How did the Stormlands and Ironborn rule the Riverlands?

[Canon Claude]

It's true that both regions have a far more martial culture than the people of the Riverlands, but they both have inferior numbers. According to the accepted numbers, the Stormlands and Ironborn have about ten or twenty thousand fewer men than the Riverlands' forces. Plus they'd also have the common people who'd serve as militia in the aim to free themselves. Surely if the rivermen had united, they could have overthrown their oppressors? And yet the stormlords can hold the Riverlands for three hundred years??

[LindsayLohan]

"Ours is the Fury" vs. "Let me catch that fish, homie."

[Protagoras]

Short answer: Crab-mentality. Long answer: Well...

First, the riverlands defences are lacking. You could in theory hold and use the ford against the invaders, but large parts are very easily to conquer. And if you meet the Ironborn, you have sort of lost your advantage from rivers instantly. Regardless what some might think, natural geography is huge.

Secondly, they, like the Vale, they  was one of the few kingdoms who got the full force of the Andals, and since they have few natural defences they were unable to hold. So their kingly dynasty was shattered. And when one dynasty had fallen, others can too. The Starks, Arryns and Lannisters have been around for a while, building up themselves as an institution, but if you have to rebuild the kingdom again and again and again its going to be hard to get the right legitimity. You need roots and tradition for effective control. This lack of central authority when you are in war is important. Just look at the (lack) of stormland resistance from Aegon and Golden company.

Thirdly, we have these instances:

The realm that Benedict the Bold had knitted together was torn asunder once again, and a hundred years of conflict saw petty kings from the Houses Blackwood, Bracken, Vance, Mallister, and Charlton contending with one another for supremacy.

This led Raventree to rise against him, for the Blackwoods had never accepted the Seven. The Vances of Atranta and the Tullys of Riverrun joined them in rebellion. King Humfrey and his loyalists, supported by the Swords and Stars of the Faith Militant, were on the point of crushing them when Lord Roderick Blackwood sent to Storm's End for aid. His lordship was tied to House Durrandon by marriage, as King Arlan had taken one of Lord Roderick's daughters to wife, wedding her by the old rites beneath the great dead weirwood in Raventree's godswood.

But when Lady Agnes advanced upon the ironborn, her belligerent neighbor Lord Lothar Bracken fell upon her rear with all his strength and put her men to flight. Lady Agnes herself and two of her sons were captured and delivered to King Harwyn, who forced the mother to watch as he strangled her boys with his bare hands.

The Riverlords are disunited (maybe because of point 2) and have a tendency to fall into civil wars to solve their crises. In addition to this, they have no problem to side behind whatever invader that can help them from a short-term perspective (that riverlords were willing to so easility swear fealty to Robb as king, despite Tully being their overlord, is another example), ignoring the common logic that the stranger "helping" you might have hidden motives (who doesn´t "get" that House Durrendon or House Hoares might have plans on their own). The riverlords are in practice lords that lives close to water and have neighbors they don´t like. Calling them "The riverlords" present them with a false sense of unity. And we have several houses with ambitions - Bracken, Blackwood, Frey, Tully, Darry. Maybe even Vance and Mallister. And then Harrenhal, the greatest castle, can´t get a long-lived dynasty settling it.

Fourth, they (because of the power-struggles above) think short-sighted when it comes to economy too.

There has never been a city in the riverlands, strange as that might seem (though large market towns are common), likely because of the fractious history of the region and a tendency for the kings of the past to refuse the charters that might have given some Saltpans or Lord Harroway's Town or Fairmarket leave to expand.

They see economic centras and developments as threats to their own power! This both hurt loyalty (why support someone who work against your expansion) and economy (Those neighbors will make those charters, gaining an economical advantage regionwise).

[Floki of the Ironborn]

The Ironborn only ruled for a short amount of time, presumably having to work hard to put down rebellions as they came up. Same with the stormlords. Ruling the riverlands wasn't a stroll in the park. And as has already been established, it only worked because the riverlords were divided and distrusted each other. Had they united into a single rebellion, like Robar Royce II, then things might have turned out much differently.

[Ralphis Baratheon]

5 hours ago, Protagoras said:

Short answer: Crab-mentality.

This seems the most likely. They lacked unity because two of the strongest houses didn't want to see the other become "Kings." 

[Lord Wraith]

8 hours ago, Canon Claude said:

It's true that both regions have a far more martial culture than the people of the Riverlands, but they both have inferior numbers. According to the accepted numbers, the Stormlands and Ironborn have about ten or twenty thousand fewer men than the Riverlands' forces. Plus they'd also have the common people who'd serve as militia in the aim to free themselves. Surely if the rivermen had united, they could have overthrown their oppressors? And yet the stormlords can hold the Riverlands for three hundred years??

http://towerofthehand.com/blog/2016/11/08-politics-of-seven-kingdoms-4/index.html