- "Thus the Starks took it upon themselves to unify the North... under them. They drove the pirates out of White Knife, and claimed the eastern coast. And married the Marsh King's daughter for the Neck. A Stark wrestled for Bear Island and won. Or so they say... Silly stories all. Blood and steel won the North... and the Starks had the most of both."
- ―Roose Bolton
The Unification of the North was a process that traditionally lasted for two milennias, in which House Stark gradually repelled Ironborn invaders and subdued Northern rivals, particularly House Bolton. The countless wars saw the birth of the Kingdom of the North, which would last until the War of Conquest more than 6,000 years later.
In the Age of Heroes, The North was divided between several petty kingdoms. The Winter Kings of Winterfell and the Red Kings of the Dreadfort were the most powerful of them all, and their rivalry resulted in countless wars over a period of several centuries. These wars were decidedly indecisive, and the Boltons killed several Starks in this period. The Boltons also earned themselves a sinister reputation for flaying their enemies, a practice that eventually made itself to their sigil, which displays a flayed man hanging upside down from a cross.
Internal Northern wars had the effect of opening the North for pirates and raiders as well. King Jon Stark drove Sistermen pirates from the White Knife, and his son Rickard Stark defeated the last Marsh King and brought The Neck into the fold.
Not all parts of the North were brought under Stark dominion by military means. House Manderly, an exiled house from The Reach, was given land by the White Knife in exchange for loyalty. King Rodrik Stark won Bear Island from the Ironborn in an arm wrestling match with a leader of House Hoare; the Island was subsequently gifted to House Mormont. The fealty of House Umber was won by helping them fight off wildling raiders. House Karstark came to be when Karlon Stark was given Karhold in return for putting down a rebellion.
Due to their inferior numbers vis-a-vis the Starks, the Boltons were eventually defeated. The last Red King Rogar Bolton bent the knee to House Stark, who later banned their gruesome practices. This marked the birth of the Kingdom of the North.
The Boltons bent the knee just in time for the North to face the Andal Invasion united. The Starks, the Boltons and other Northern houses faced Andal invaders at Moat Cailin countless times, until the Andals eventually gave up their attempts to conquer the North. This joint effort did not result on harmony between the Northern houses; the Boltons rebelled against Stark rule several times, though the Starks put down every revolt.
The bad blood between the Starks and the Boltons persists 6,000 years later. The Boltons were one of the orchestrators behind the Red Wedding, which saw the fall of the newly-reborn Kingdom of the North and the death of King Robb Stark. The Boltons were defeated for good in the subsequent Battle of the Bastards, in which the remainder of the Stark family retook Winterfell and killed Ramsay Bolton, the last Bolton alive.