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“Aegon saw that three hundred years ago when he stood where we are standing. They painted this table at his command. Rivers and bays they painted, hills and mountains, castles and cities and market towns, lakes and swamps and forests… but no borders. It is all one. One realm, for one king to rule alone.” –A Storm of Swords, Davos IV
Aegon Targaryen. Aegon the Conqueror. Aegon the Dragon. Few men changed Westerosi history as much as the dragonlord with the dramatic vision: one continent, one realm, one king. In a continent marked with wars from Dorne to the Wall, the notion that one man could control all of Westeros was nothing short of a fantasy. There are three major ethnic groups, three dominant religions, at least eight distinct regions each with their own cultures and subcultures littering a continent roughly the size of South America. Holding such territory under a single authority would be almost impossible without a way to project the authority needed, though this did not stop kings from trying. Arlan III Durrandon, the Storm King of that time, extended his reach to the Riverlands, though every generation a Riverlander would attempt to overthrow him. The Hoare kings of the Iron Islands would do the same three centuries after the Stormlander conquest, invading the territory with the help of Riverlander infighting, defeating the overeager Arrec Durrandon (who marched ahead of his baggage train, a disaster that spelled doom for many generals both in A Song of Ice and Fire’s world and our own), and placing himself as King of the Riverlands, subjecting the Riverlanders to thralldom for three generations.
The Hoare kings could boast that they controlled the largest swath of territory in Westeros, but a new invader was rising in the east, and would conquer almost the entire kingdom, in fire and blood.
Wars and Politics of Ice and Fire 