When Hoster Tully fleeing King's Landing, he went back to Riverrun to rally the River lords to fight against Godbrand Greenhand and his allies. He also sent missives to his 1st cousin once removed, Queen Sansa of the North, as well as the Lords of the Stormlands, Dorne, Westerlands and Vale, hoping that united they would be able to take down Godbrand's Alliance with the Iron Islands and Crownlander lords.
To project his legitimacy and fulfil his ambitions, he crowns himself King of the Andals and the First Men.
Things start to go wrong almost immediately:
- Dorne and the Stormlands declare neutrality. Being separated from the Riverlands by Godbrand's massive empire, one could argue they had little choice in the matter. Another point is the fact that the Princess of Dorne was a friend of Olenna Greenhand, Godbrand's wife. Lord Davos Baratheon thought Hoster had little chance of winning, having received reports of Godbrand's powerful army.
- The Arryns were in a similar position to when the War of the Five Kings was waged. Lord Robin Arryn had died five years earlier and his son Oswin was only 10. Regent of the Vale was Shiera Arryn, born Velaryon, who held a grudge against Hoster personally, due to earlier quarrels, back when he was Master of Laws. Hoster's hopes of the other Vale lords pressing to join him were unfounded, because they chose to heed the Lady Regent's orders.
- The new Lord of Casterly Rock, Loreon Lannister does not answer Hoster's summons and instead proclaims himself King of the Rock.
- Sensing the way the coming war was going to go, Sansa Stark chooses to send her troops to secure the northern parts of the Riverlands instead.
The war comes quicker than Hoster predicts, when Godbrand sends his forces to attack the Riverlands and King Daenar Greyjoy attacks Seagard. Surrounded and with nowhere to go, Hoster makes several moves to try to rally the disunited Riverlords, such as naming himself King of the Trident.
Not a moon later, Hoster is greatly outnumbered at a battle near Riverrun, where his army is butchered. Half dead, he demands single combat from Godbrand who slays him with ease.
In the aftermath, the Reach Emperor sends requests to all the sides to gather and discuss. The result is the Great Council of 347 AC:
- With House Tully practically extinct, the elder Lord Edmure dying from a stroke shortly after the last battle, the Riverlands are left leaderless. The Great Lords gathered chose to divide the region among themselves.
- The southern part is given to the Reach, the western part to the Lannisters, the northernmost lands to the North and Seagard to the Ironborn. These are practically the territories already occupied by the respective forces.
- Riverrun is given to the Westerlands, but Prince Jon Stark, the second son of Queen Sansa, is made its lord.
- To strenghten his house and protect himself from the Reachmen, King Loreon allies with the North and marries Queen Sansa's youngest daughter Lyanna.
- The other regions of Westeros (Stormlands, Vale, Dorne), whose lords are present, are declared independent kingdoms with rulers: Oswin III Arryn of the Vale, Princess Nymeria III Martell of Dorne and King Davos I Baratheon of the Stormlands)
- An yearly convention of monarchs is created, where the various monarchs of Westeros will gather to discuss ongoing issues and resolve crises.
- This marks the official disestablishment of the unity of the Seven Kingdoms, imposed during Aegon's Conquest.
The Division of the Riverlands, according to the Great Council of 347 AC: White - Kingdom of the North; Black - Kingdom of Mountain and Vale; Red - Kingdom of the Rock; Green - Empire of the Reach; Orange - Principality of Dorne; Brown - Kingdom of the Iron Islands; Yellow - Kingdom of the Stormlands
The Current rulers of the Seven Kingdoms:
Kingdom of the North - Sansa I Stark; Heir Prince Eddard Stark;
Kingdom of Mountain and Vale - King Oswin III Stark; Regent Shiera Velaryon;
Kingdom of the Rock - King Loreon VI Lannister;
Empire of the Reach - Emperor Godbrand Greenhand; Heir Prince Imperial Godric Greenhand;
Kingdom of the Stormlands - Storm King Davos I Baratheon; Heir Princess Argella Baratheon;
Kingdom of the Iron Islands - King Daenar I Greyjoy; Heir Princess Asha Greyjoy;
Principality of Dorne - Princess Nymeria III Martell; Heir Prince Maron Martell.