Long story short, the Iron Throne (Crownlands + Stormlands, Reach and Dorne) attacked the Kingdom of the Trident (Riverlands + Vale and Westernlands) over a De jure county claim.
The Iron Throne won, and got the county, but in the outcome of the war several vassals of the Iron Throne (who had been independent during the war because of the Mega war system) became vassals of the Kingdom of the Trident.
The Iron Throne won, and got the county, but in the outcome of the war several vassals of the Iron Throne (who had been independent during the war because of the Mega war system) became vassals of the Kingdom of the Trident.