Stark-Lannister alliance - could it work?

[devilish]

Time period: War of 5 kings
Major players: Robb Stark and Tywin Lannister
Event: Jamie is captured, Robb is crowned king in the North

Summary:

Robb is undefeated but he’s only got a 20k army. Tywin has a bigger army but has to contend against multiple enemies. Both are in a deadlock. Robb lack the men to invade KL and take on the Lannisters. Tywin can’t take on Robb without risking Renly’s backlash. Both risk to end up being taken down by Renly whose got superior forces and won’t accept anything but them bending the knee.

Terms

Robb demands
-    The crown will issue an official statement that the crown and the North fell victim to a devious plan by the Baratheon brothers/usurpers to pit the Starks and the crown against one another. Ned’s honour is immediately restored and his bones are sent to the North in full pomp. 
-    Following the unnecessary execution of the warden of the North and war veteran Ned Stark. King Joffrey will abdicate from his duty as king and he will take the black. King Tommen will take his place
-    King Tommen will release Robb from his vows with the Freys. The rats erm Freys will be given Harrenhal as reparation. 
-    The Lannisters will leave the Riverlands once and for all. 

Robb’s concessions
-    Robb bends the knee to King Tommen but will retain the title of Prince. 
-    Robb will marry Princess Myrcella, meanwhile Tommen will marry Sansa. Hence they will quash any nasty rumours surrounding Robert Baratheon’s children.
-    Jamie is returned to the Lannisters. Cersei marries Edmure Tully
-    The North and the Riverlands will join the royalist army. 

 Comments:
At that point, Tywin knows very well that his grandson is mad. There’s no other way to explain him executing Ned. By sacrificing him, he’s making sure that the Baratheon-Targ dynasty will survive and prosper + he will be bringing two powerful regions back to the fold. 
Robb on the other hand must know that Lysa won’t send the knights of the Vale to his support. 20k will not get him very far either especially since he also have to defend the riverlands.  
 

[Lion of the West]

After Eddard dies I think its impossible for an alliance between Robb and Tywin. Neither can back down from a confrontation and keep their credibility and authority and even before Eddard's opinion of Tywin was to poisonous for there to have been a working alliance. Under Sansa or Bran perhaps something could have been struck with a Lannister leadership like Daven or Damion, provided there hasn't been additional bloodshed between the Houses. But that's probably it.

[norwaywolf123]

19 minutes ago, devilish said:

Robb demands
-    The crown will issue an official statement that the crown and the North fell victim to a devious plan by the Baratheon brothers/usurpers to pit the Starks and the crown against one another. Ned’s honour is immediately restored and his bones are sent to the North in full pomp. 
-    Following the unnecessary execution of the warden of the North and war veteran Ned Stark. King Joffrey will abdicate from his duty as king and he will take the black. King Tommen will take his place
-    King Tommen will release Robb from his vows with the Freys. The rats erm Freys will be given Harrenhal as reparation. 
-    The Lannisters will leave the Riverlands once and for all. 

1 + 2: The baratheon plot sounds sound. Does it have to be after Neds execution? Maybe if Robb knew of the danger to the north he woud return for a bad deal anyway?

4: What do you mean? Will the riverlands be subject to House stark, house tully or directly to the ironthrone. Is house tully subject to house stark?

19 minutes ago, devilish said:

Robb’s concessions
-    Robb bends the knee to King Tommen but will retain the title of Prince. 
-    Robb will marry Princess Myrcella, meanwhile Tommen will marry Sansa. Hence they will quash any nasty rumours surrounding Robert Baratheon’s children.
-    Jamie is returned to the Lannisters. Cersei marries Edmure Tully
-    The North and the Riverlands will join the royalist army. 

1. The martells are called prince/princess becouse of their rhoynish roots, the north have always been seen as kings or lords, so this makes no sense to me.

19 minutes ago, devilish said:

 Comments:
At that point, Tywin knows very well that his grandson is mad. There’s no other way to explain him executing Ned. By sacrificing him, he’s making sure that the Baratheon-Targ dynasty will survive and prosper + he will be bringing two powerful regions back to the fold. 
Robb on the other hand must know that Lysa won’t send the knights of the Vale to his support. 20k will not get him very far either especially since he also have to defend the riverlands.  
 

This would be a fantactic deal for Tywin! Robb would lose face me thinks.

[Kytheros]

It's far more plausible that Robb would ally or bend the knee to one of the Baratheon brothers (Stannis or Renly) than he'd ally with the Lannisters.

It's more likely that Robb finds common cause with the Martells than the Lannisters.

Heck, it's more likely that Balon Greyjoy would accept Robb's offer, than any sort of alliance between Stark and Lannister. Especially after Ned dies, but there was never a chance for a Stark/Lannister alliance anyways.

 

Heck, it's far more plausible that Stannis brings Robb back into the fold via betrothing Shireen to Robb. I'm not sure why that was never on the table.

 

 

I doubt Robb would have allied with the Lannisters in the face of a Targaryen Return at that point. Maybe in the face of the White Walkers getting past the Wall, but anything less? Not likely.

[Retrograde]

5 minutes ago, Kytheros said:

It's far more plausible that Robb would ally or bend the knee to one of the Baratheon brothers (Stannis or Renly) than he'd ally with the Lannisters.

It's more likely that Robb finds common cause with the Martells than the Lannisters.

Heck, it's more likely that Balon Greyjoy would accept Robb's offer, than any sort of alliance between Stark and Lannister. Especially after Ned dies, but there was never a chance for a Stark/Lannister alliance anyways.

 

Heck, it's far more plausible that Stannis brings Robb back into the fold via betrothing Shireen to Robb. I'm not sure why that was never on the table.

 

 

I doubt Robb would have allied with the Lannisters in the face of a Targaryen Return at that point. Maybe in the face of the White Walkers getting past the Wall, but anything less? Not likely.

I agree. Robb wouldn't ally with Lannisters. He'd rather die

[devilish]

18 minutes ago, LionoftheWest said:

After Eddard dies I think its impossible for an alliance between Robb and Tywin. Neither can back down from a confrontation and keep their credibility and authority and even before Eddard's opinion of Tywin was to poisonous for there to have been a working alliance. Under Sansa or Bran perhaps something could have been struck with a Lannister leadership like Daven or Damion, provided there hasn't been additional bloodshed between the Houses. But that's probably it.

Things do get complicated after that. However, I still feel that there was room for negotiation. Tywin wants to consolidate his legacy. With enemies at all sides he simply lack the men and the heir to do so (especially if Robb decides to chop Jamie's head off). Even if he does manage to win (which is very unlikely at that point) the rumours surrounding Joffrey + the boy's stupidity will probably ruin everything. No one has more experience in recognising and in dealing with mad kings then the old lion. 2 regions + a 20k army in exchange of Aerys V2 is quite a fair deal

Robb on the other hand simply lack the men to invade KL. His 'King in the North and the Riverlands' bravado means that he's stuck on a land that is very difficult to defend with barely with barely any army to do so. 20k might be a solid army if tucked at the right end of Moat Cailin but down South its peanuts. Both Renly and Tywin could easily wipe that out if they put their full effort on it. Ensuring his family safety, getting justice over the man who killed Ned + returning home with enough hostages to ensure peace is quite a fair deal.

 

 

[devilish]

17 minutes ago, norwaywolf123 said:

1 + 2: The baratheon plot sounds sound. Does it have to be after Neds execution? Maybe if Robb knew of the danger to the north he woud return for a bad deal anyway?

4: What do you mean? Will the riverlands be subject to House stark, house tully or directly to the ironthrone. Is house tully subject to house stark?

1. The martells are called prince/princess becouse of their rhoynish roots, the north have always been seen as kings or lords, so this makes no sense to me.

This would be a fantactic deal for Tywin! Robb would lose face me thinks.

1+2 everything went tits up following Ned's execution. If Joffrey didn't stupidly executed Ned then the Starks would probably seal a deal with the Lannisters in exchange for Sansa, Arya and Ned.

4: House Tully will be subject to the Lannisters. However, with Cersei at Riverrun, Edmure can ensure that the Lannisters won't seek any form of retribution as soon as the Starks return home. 

1: Its a way for Robb to get out of this mess with his head held high. 

[norwaywolf123]

3 minutes ago, devilish said:

1+2 everything went tits up following Ned's execution. If Joffrey didn't stupidly executed Ned then the Starks would probably seal a deal with the Lannisters in exchange for Sansa, Arya and Ned.

4: House Tully will be subject to the Lannisters. However, with Cersei at Riverrun, Edmure can ensure that the Lannisters won't seek any form of retribution as soon as the Starks return home. 

good points

3 minutes ago, devilish said:

1: Its a way for Robb to get out of this mess with his head held high. 

i still don't see it. It is just a title. 

[devilish]

12 minutes ago, Retrograde said:

I agree. Robb wouldn't ally with Lannisters. He'd rather die

There's no way Robb could ensure the safety of his sisters + keep his crown. He simply lacked the numbers to do so. Renly might have allowed him to bend the knee and suffer no repercussions whatsoever but there's no way he could save Sansa in KL. Cersei would make sure that she would die the moment KL is stormed. Meanwhile once the Tyrell joined forced with the Lannisters the war was over with or without Stannis backing.

The plan I've mentioned would have secured, the Stark's, Edmure's and Sansa's future + it would get rid of a mad king Mark II. With Cersei in Riverrun and Mycella in Winterfell there's no way the Lannisters would hurt Sansa in any way. 

 

Not to forget that the ceasefire would facilitate Arya's return to her family. 

[devilish]

4 minutes ago, norwaywolf123 said:

good points

i still don't see it. It is just a title. 

Maybe, but nobility just love titles. I've seen brothers (who originate from a noble family) skinning each other alive on a frigging title that means nothing. 

[norwaywolf123]

37 minutes ago, devilish said:

Maybe, but nobility just love titles. I've seen brothers (who originate from a noble family) skinning each other alive on a frigging title that means nothing. 

IRL?

[The Sunland Lord]

1 hour ago, devilish said:

Time period: War of 5 kings
Major players: Robb Stark and Tywin Lannister
Event: Jamie is captured, Robb is crowned king in the North

A peace agreement could work, but not an alliance. 

1 hour ago, devilish said:

Terms

Robb demands
-    The crown will issue an official statement that the crown and the North fell victim to a devious plan by the Baratheon brothers/usurpers to pit the Starks and the crown against one another. Ned’s honour is immediately restored and his bones are sent to the North in full pomp. 
-    Following the unnecessary execution of the warden of the North and war veteran Ned Stark. King Joffrey will abdicate from his duty as king and he will take the black. King Tommen will take his place
-    King Tommen will release Robb from his vows with the Freys. The rats erm Freys will be given Harrenhal as reparation. 
-    The Lannisters will leave the Riverlands once and for all. 

-The first point is reasonable.

-Cersei would never let Joffrey take the black. Also Tywin will not allow himself the Lannisters look that weak by sending one of their own to the Wall.

-If we're talking about changing turn of events, Robb may want to marry Roslin after the war and cold-headed. Remember at the time there's no Jeyne Westerling. Plus, with the peace agreement signed the Freys have no back up from Lannisters to overthrow Tullys and Boltons don't have the back up to overthrow Starks. No reason why Harrenhal would be given to Freys. 

-Fourth point is reasonable.

1 hour ago, devilish said:

Robb’s concessions
-    Robb bends the knee to King Tommen but will retain the title of Prince. 
-    Robb will marry Princess Myrcella, meanwhile Tommen will marry Sansa. Hence they will quash any nasty rumours surrounding Robert Baratheon’s children.
-    Jamie is returned to the Lannisters. Cersei marries Edmure Tully
-    The North and the Riverlands will join the royalist army. 

-Since circumstances are exceptional, Robb wouldn't risk bending to Joffrey in King's Landing (OK, or Tommen), but would recognize him as the rightful King with an official statement, and renounce his title as a King and become a Warden of the North. This would please both sides, Robb keeps his head for sure and Lannisters can count the North as a part of the Seven Kingdoms.

-Marriages won't fly after Ned's beheading. "Remember when your brother ordered the beheading of my father?" This would be the reason why not. 

-Jaime is exchanged with Sansa, presumably Arya is found to be taken back to WF instead for a hostage, maybe both sides participate in searching for her. Also, every prisoner on both sides is exchanged. Cersei/Edmure wouldn't work if you ask me. She was the reason his nephew was pushed from a tower and this is now a common knowledge (if I'm not mistaken).

-North and Riverlands can't fight along with their yesterday's enemies. Both armies won't build mutual trust overnight, after many battles already took place and too much blood already fell. This is where Lannisters pay the real price for starting the war. Can't count on those troops, only not having them as enemies. 

With the Baratheon brothers' events turning the way they turned, Lannisters would defeat Stannis along with Tyrells anyways, since Robb didn't participate in the Battle of Blackwater bay. So this would stay the same.

 

 

[Lord Lannister]

This has a lot of large obstacles to overcome. Tywin might be interested in reaching an accommodation with Robb, but Cersei and Joffrey definitely were not. As a Clash of Kings clearly showed, there was a definite misalignment in both political and military strategy between King's Landing and the Lannister forces in the field. For this to have any chance, Robb would have to present his independence demands directly to Tywin rather than to King's Landing.

Tywin of course isn't going to agree to them, but might see them as a starting point for negotiations. The terms you suggest might be reasonable, from Tywin's point of view, save for the abdication of Joffrey. That could very well represent a loss of face that is intolerable, coupled with the detail that Joffrey and Cersei would never agree to it and Tywin would have to overthrow them with force to enforce this.

Which brings us back to Robb. His reaction to the counteroffer is almost certainly going to be indignation. Tywin and company get off freely for burning the Riverlands, He's already engaged to the Frey girl at this point, which for irony would be the primary reasoning behind his declining, and the Northern lords aren't going to tolerate submitting to the Lannisters, just to get entangled in a war against the Baratheons.

Even if Catelyn for some reason were to encourage Robb to accept this, we all know Robb has no problem with going against his family's advice. Especially since Renly would seem to be a better candidate to reach an accommodation with. His offer was actually better.

[Kytheros]

There's no chance that Robb would make a deal with Tywin.

There is a reasonably strong possibility that Robb could and would make a deal with Renly or Stannis. The only real obstacles to dealing with Stannis or Renly would be pride and stubbornness.

 

The right deal with Stannis - Robb marries/gets betrothed to Shireen - would be the strongest and most acceptable (to everyone involved, except the Freys). Stannis can be trusted.

Making a deal with Renly, while possible, is a bit trickier to finesse appropriately, and is probably weaker in the long run than a deal with Stannis. But still quite doable.

[devilish]

@The Sunland Lord

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Cersei would never let Joffrey take the black. Also Tywin will not allow himself the Lannisters look that weak by sending one of their own to the Wall.

 

 

- Cersei is mare to breed for Tywin. The issue here would be the Warden of the West. With Renly able to lead a 60k-80k army to war can Tywin  really afford turning down the demands of a 20k enemy whose willing to become a friend? Especially since that Robb has the golden boy in 'custody'. Is it worth to risk being encircled and destroyed and have your entire family wiped out just to preserve the honour of a mad king? Or is it worth to sacrifice that incompetent boy in the name of a long Lannister dynasty with marital ties stretched at the 4 corners of Westeros?

 

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-If we're talking about changing turn of events, Robb may want to marry Roslin after the war and cold-headed. Remember at the time there's no Jeyne Westerling. Plus, with the peace agreement signed the Freys have no back up from Lannisters to overthrow Tullys and Boltons don't have the back up to overthrow Starks. No reason why Harrenhal would be given to Freys.

 

 I don't think Robb wanted to marry a Frey. Sure he didn't want to marry Myrcella either. There again with Myrcella at Winterfell and Cersei at Riverrun his uncle's and his sister's lives were secure. The lions would never risk hurting on of their own.

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-Since circumstances are exceptional, Robb wouldn't risk bending to Joffrey in King's Landing (OK, or Tommen), but would recognize him as the rightful King with an official statement, and renounce his title as a King and become a Warden of the North. This would please both sides, Robb keeps his head for sure and Lannisters can count the North as a part of the Seven Kingdoms.

 That's metaphorically speaking. Once Joffrey has taken the black, Robb's and Tywin's could meet in Harrenhal. A fast marriage between Cersei, Mycella, Robb and Edmure would follow with Jamie released soon afterwards. Tywin's and Robb would then march south were they can take on Renly and the Tyrells. There's no need for Robb to actually bend the knee to the king.

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-Marriages won't fly after Ned's beheading. "Remember when your brother ordered the beheading of my father?" This would be the reason why not. 

The Kings of Winter had married off the daughters of the same Lords that they had just crushed. The Baratheon bastard married the daughter of the Storm king after his half brother killed her daddy and his men had paraded her naked in chains.

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Cersei/Edmure wouldn't work if you ask me. She was the reason his nephew was pushed from a tower and this is now a common knowledge (if I'm not mistaken).

No one knows that. Also note that Tywin considered marrying the lioness to Theon despite her first husband had wiped half of the Greyjoy's clan

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North and Riverlands can't fight along with their yesterday's enemies. Both armies won't build mutual trust overnight, after many battles already took place and too much blood already fell. This is where Lannisters pay the real price for starting the war. Can't count on those troops, only not having them as enemies.

That is why Cersei and Myrcella will be in Winterfell/Riverrun. If the lion hesitates they will end up very very badly

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With the Baratheon brothers' events turning the way they turned, Lannisters would defeat Stannis along with Tyrells anyways, since Robb didn't participate in the Battle of Blackwater bay. So this would stay the same

The lion won because Stannis used blood magic against his brother. That's called a fluke. In normal circumstances Renly would have moved his horde to KL and Tywin would have came late to the show as Robb 20k would have harassing him all the way. Sansa, Cersei, Joffrey and Tommen would lose their heads. Jamie would die soon afterwards, the lion and the wolf would rip each other up and the Stag would wipe away the victor with ease. There would have been 1 winner out of this whole thing and its neither a lion nor a wolf

 

[devilish]

34 minutes ago, Kytheros said:

There's no chance that Robb would make a deal with Tywin.

There is a reasonably strong possibility that Robb could and would make a deal with Renly or Stannis. The only real obstacles to dealing with Stannis or Renly would be pride and stubbornness.

 

The right deal with Stannis - Robb marries/gets betrothed to Shireen - would be the strongest and most acceptable (to everyone involved, except the Freys). Stannis can be trusted.

Making a deal with Renly, while possible, is a bit trickier to finesse appropriately, and is probably weaker in the long run than a deal with Stannis. But still quite doable.

Renly would have been the easier option. The young stag is a politician, he will forgive Robb's impertinence and will allow him to join the fray. There again even the young stag 80k army wouldn't be able to save Sansa. Once the stag breaches KL gates then Cersei would make sure that the little bird would die.

Stannis has nothing to offer to Robb irrespective if he's Lord of Dragonstone or LP of the Stormlands. The former scenario would mean that Robb will have to bend the knee at a 'mighty' king who brings a measly 3k army to the fray. At the latter scenario Robb's army will be boosted to 40k but Tywin's army will be boosted by at least 60k (the Tyrells).

 

 

[Kytheros]

21 minutes ago, devilish said:

Renly would have been the easier option. The young stag is a politician, he will forgive Robb's impertinence and will allow him to join the fray. There again even the young stag 80k army wouldn't be able to save Sansa. Once the stag breaches KL gates then Cersei would make sure that the little bird would die.

Stannis has nothing to offer to Robb irrespective if he's Lord of Dragonstone or LP of the Stormlands. The former scenario would mean that Robb will have to bend the knee at a 'mighty' king who brings a measly 3k army to the fray. At the latter scenario Robb's army will be boosted to 40k but Tywin's army will be boosted by at least 60k (the Tyrells).

 

 

Stannis has the better claim/position in the succession than Renly. Also, by betrothing Shireen to Robb, it neatly bypasses the stubborness and pride on both sides. Robb, as King in the North, isn't bending the knee to the Iron Throne in subservience and loss of independence, but to his father-in-law, whom his children will follow - Robb's heir with Shireen is the heir to everything Stannis has. With some negotiations, when Stannis dies, and Shireen/Robb take over (it's unclear whether Shireen would be a Regnant or Conveyant), and then their heir takes over, the North isn't kneeling to the Iron Throne, but to the King in the North who also happens to sit on the Iron Throne. That's ... pretty huge for the North, and it's a deal that Stannis would probably be willing to accept. Oh, sure, there'd be need to iron out the details, but in large part? It's a pretty easy deal - especially since Shireen isn't going to be winning any beauty contests.

In addition, a deal with Stannis might well strengthen his position enough that if/when Renly dies, the Stormlanders who were with him don't go with the Tyrells over to the Lannisters, and might even cause the Tyrells to not instantly join with the Lannisters, and instead see what happens before deciding which way to jump.

 

Making a deal with Renly, on the other hand, requires a lot more work, and giving up a lot, in exchange for a largely symbolic change of title. Sure, it's probably still an accomplishable deal, but it's a much tougher sell, and takes a lot longer to work out.

 

 

Making a deal with Tywin, on the other hand, is still unthinkable. At this point, IIRC, Robb's been made aware of the suspicions that the Lannisters killed his uncle Jon Arryn, knows they killed his father (after saying he'd be allowed to take the black), and has every reason to believe that Tyrion Lannister tried to have Bran killed while Bran was in a coma.

Plus, Robb's been busy beating the Lannisters where and when he's met them in battle.

 

Also, sure, Sansa probably dies to prevent her rescue if/when King's Landing gets taken from the Lannisters, but there's no way Robb's getting her back anyways. Hoping she doesn't die when King's Landing gets taken is probably Robb's best bet for Sansa's recovery, realistically speaking. The Lannisters can't give Sansa back, otherwise there's absolutely no reason for Robb to have any sort of restraint in killing the Lannisters, as she's the only possible leverage they have left. I suppose it's remotely possible that the Lannisters might be willing to deal Sansa for Jaime, but that's iffy, and even if they are, any exchange is liable to be a trap.

[devilish]

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Stannis has the better claim/position in the succession than Renly. Also, by betrothing Shireen to Robb, it neatly bypasses the stubborness and pride on both sides. Robb, as King in the North, isn't bending the knee to the Iron Throne in subservience and loss of independence, but to his father-in-law, whom his children will follow - Robb's heir with Shireen is the heir to everything Stannis has. With some negotiations, when Stannis dies, and Shireen/Robb take over (it's unclear whether Shireen would be a Regnant or Conveyant), and then their heir takes over, the North isn't kneeling to the Iron Throne, but to the King in the North who also happens to sit on the Iron Throne. That's ... pretty huge for the North, and it's a deal that Stannis would probably be willing to accept. Oh, sure, there'd be need to iron out the details, but in large part? It's a pretty easy deal - especially since Shireen isn't going to be winning any beauty contests

You need to see things from the North’s perspective. They are isolationists and had only marched South because Robb’s dad, sisters, Uncle and grandpa were in danger. They can’t care less who sits on the iron throne. All they want is justice and to make sure that as many family members as possible remain safe.

The Vale’s disinterest in their war is a problem. The North simply lacks the men to keep the Riverlands safe. If Tywin is able to solely focus on the war at the Riverlands then the Northern army will be wiped out.  Same thing can be said about Renly whose got 3-4 times the men Robb has.

Stannis might have the claim to the IT but he’s as useful as bull tits. At the beginning of the campaign he only holds a 3k army. After he commits kinslaying his army is boosted to 20k but by doing so he was able to push the Tyrells in bed with the Lannisters ending the war once and for all. Even if united under the same banner Robb and Stannis could never win the war. They simply lack the men. 

Shireen suffered from greyscale, an infective and lethal disease. Currently it’s in remission but such sickness can easily return and end up infecting Robb. Not to forget that there are question marks whether she can produce a heir.  There’s no way Robb would risk that especially since her daddy hardly has the troops to conquer the darn place.

My point is relatively easy to understand. If Robb wants to win the war then he will bend the knee to Renly. He’s got superior forces and if Robb adds his 20k to his 60k-80k then the war is over. If he wants as many family members back and safe then he makes a deal with Tywin. There’s no way Sansa can survive the imminent sacking of KL.

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In addition, a deal with Stannis might well strengthen his position enough that if/when Renly dies, the Stormlanders who were with him don't go with the Tyrells over to the Lannisters, and might even cause the Tyrells to not instantly join with the Lannisters, and instead see what happens before deciding which way to jump.

I think that you’re underestimating The Reach and the Tyrells.

 Let me give you an idea. At the beginning of the war of 5 kings, Robb’s army leaves the North with an army of around 18-19k. House Frey adds 4k to it raising the army to around 20k-22k. At the battle of blackwater’s bay Stannis Baratheon had around 20k. 

The Lannisters raised two armies one of which was 20k strong and was led by Tywin Lannister and another army was 14k-15k strong and it was led by Jamie Lannister. 

According to Tyrion Lannister the Tyrells were able to surround KL with a 70k horde. Highgarden alone can raise a 20k army in a month (Loras Tyrell) while House Hightower can raise a 9k army
That says part of the story. The Tyrell army is led by Randyll Tarly, a general btw who actually wins wars on the battlefield even against the only Baratheon who actually fought during Robert’s rebellion. 

But lets have a look at Mace’s alliances within the Reach.

Mace Tyrell is married to a Hightower and his mother is a Redywne. He is also best bro to Randyll Tarly who chooses him ahead of his wife’s family. Mace also have 2 unmarried children (3 if you add Margaery) which can easily be used as pawns to consolidate power. That means that  Mace has full control over the bannermen that count. If he says jump then his bannermen/relatives will say how high.

In matter of fact, not even good old Tywin with his diplomacy prowess was able to make inroads on that regard.

What about the Tyrell’s relationship with Stannis?
a-    They nearly starved him to death. Stannis is a man who can hold grudges.
b-    He went on marrying a Florent, the only family with equal claim to the Reach as them that the Tyrells hasn’t accounted for. 
c-    He killed Mace’s son in law through blood magic.

Conclusion:

Mace Tyrell can wipe the floor with both Stannis and Robb with or without the Lannister’s help (preferably with their help as that gives the Tyrells legitimacy + a 30k boost). There’s no way Mace would allow Stannis to get on the IT either.

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Making a deal with Tywin, on the other hand, is still unthinkable. At this point, IIRC, Robb's been made aware of the suspicions that the Lannisters killed his uncle Jon Arryn, knows they killed his father (after saying he'd be allowed to take the black), and has every reason to believe that Tyrion Lannister tried to have Bran killed while Bran was in a coma.

Plus, Robb's been busy beating the Lannisters where and when he's met them in battle.

 

Also, sure, Sansa probably dies to prevent her rescue if/when King's Landing gets taken from the Lannisters, but there's no way Robb's getting her back anyways. Hoping she doesn't die when King's Landing gets taken is probably Robb's best bet for Sansa's recovery, realistically speaking. The Lannisters can't give Sansa back, otherwise there's absolutely no reason for Robb to have any sort of restraint in killing the Lannisters, as she's the only possible leverage they have left. I suppose it's remotely possible that the Lannisters might be willing to deal Sansa for Jaime, but that's iffy, and even if they are, any exchange is liable to be a trap.

 

As said Robb has zero Southern ambitions and he also know he lacks the men to defend the Riverlands let alone get his sisters back (he doesn’t know Arya has escaped).  Things get worse later on with the greyjoys invasion of the North. The North is sealed, Bran and Rickon are thought to be dead and the all remaining Starks are either in Lannister hands or sitting ducks at the undefensable Riverlands.

Now if he wants to win the war then the most plausible option would have been to bend the knee to Renly. If he wants his sisters alive, Ned’s bones back and to secure Uncle Edmure status then the only option would be to seal a deal with Tywin. Sure, bending the knee to a lion hurts.

However if 

a)    they can get the revenge on the very man who caused most of this misery by sending him to the wall (ie from Joffrey’s perspective it’s a fate worse than death especially  since he will be under the commands of either Ned’s no 1 fanboy Lord Commander Mormont or Ned’s bastard Lord Commander Jon Snow)
b- He will secure the safety of his family, Edmure and his sisters.

 

PS: There's too many hostages involved (Sansa, Jamie, Cersei, Myrcella) for the Lannisters/Starks to play dirty. 

 

[Graydon Hicks]

very good points. i agree that its a choice betweeen either winning the war and punishing the lannisters, or getting his family back safely. but if robb was in a position, say he went to treat with renly, instead of his mother, to take over the tyrell army? he is a king, he is charismatic, good looking, and with a reputation for success on the battle field against a force both numerically superior and lead by a fame accomplished military commander (did tywin deserve that particular reputation, i dont know, but most of westeros seems to think so). that gives him a lot of selling points. say he goes to treat with renly instead of his mother. think of the impression he makes on renly's army. the tyrell and stormland army is sitting pretty, far from the battleground in the riverlands, having mini tourneys and feasting instead of engaging lannister forces from the south. in marches a young king in the north, wearing battle worn armor and weapons, bearing scars, showing the evidence that his forces have actually been fighting, and winning, even with the odds against them. maybe he makes friends with renly, we see a conversation similar to what renly had with cat, and robb meets margy. margy knows renly's preferences, but keeps trying. then here is the young handsome northerner, of an age with her. maybe they become friendly, within the bounds of propriety. robb is there for the talk with stannis, and is present when the shadow kills renly, and survives himself.

now with renly dead, and the army starting to break up as stormlanders head to stannis, and reachlords starting heading home, the tyrells have this wonderful opportunity in front of them. here is a young scion of a powerful and ancient noble house, a king in his own right, with a string of victories to his name. maybe they push for robb to take command of the army, have him marry margery to seal themselves to him. with someone like olenna tyrell to provide the political advise, im sure they could find away to appease the freys, who i feel would have betrayed robb no matter what he did. and with this 60-80k army added to his own, he now has an honest chance of kicking tywin's ass up between his ears. yes sansa might die at the hands of cersie and joffrey (for the life of me i dont understand why robb never bothered to sneak a dozen or so men into the capital to get to sansa, black ops style. the concept had to exist because bronn suggested it about dealing with the eyrie.) but now he also has the means to push the war home and actually take the red keep.

[The Sunland Lord]

1 hour ago, Graydon Hicks said:

...and robb meets margy. margy knows renly's preferences, but keeps trying. then here is the young handsome northerner, of an age with her. maybe they become friendly, within the bounds of propriety.

I always thought if Robb went instead of Cat to negotiate with Renly, he'd 100% take Margaery to bed.