Wars of the First Men and the Children of the Forest

The Wars of the First Men and the Children of the Forest were a centuries-long conflict between the non-human Children of the Forest and the First Men who migrated to Westeros twelve thousand years ago.

Prelude
The vast continent of Westeros was originally inhabited by a diminutive non-human race known as the Children of the Forest. Aboriginal and few in number, they were nonetheless very woodcrafty and their wise men wielded powerful magic abilities. Twelve thousand years ago the first humans to live on Westeros, known as the First Men, migrated to Westeros from the eastern continent Essos, across a land bridge known as the Arm of Dorne.

War
The First Men began encroaching on the forests of the Children, cutting and clearing land to use them for agriculture. The First Men cut down the Children's sacred weirdwood trees, infuriating them. The Children fought back against the tide of the First Men's migration throughout Westeros, in a series of conflicts lasting two thousand years.

The Children of the Forest were never numerous, and they did not possess much technology, wielding blades and arrowsheads made out of obsidian stone. Nonetheless they were fierce fighters, knew the forests very well, and wielded magical powers. Meanwhile, the First Men were more numerous, possessed weapons and armor made of bronze, and rode horses. The greenseers of the Children of the Forest used their powerful magic to shatter the Arm of Dorne, and the ocean rose up and swallows most of the land bridge. The few remnants became an island chain between Westeros and Essos known as the Stepstones. However, the destruction of the Arm of Dorne ultimately did not stop the advance of the First Men.

The Pact
After two thousand years of conflict, the two sides had fought to a standstill. The two races tired of the centuries of bloodshed, and representatives of both groups came together on an island in the middle of a great lake in central Westeros, known as Gods Eye. There they forged a lasting peace known as The Pact: the open lands were granted to humanity, while the deep forests were to remain the undisturbed domain of the Children. The First Men were forbidden from ever cutting down a sacred weirwood tree again.

The peace between the two races would endure another six thousand years, even enduring the invasion of the White Walkers two thousand years later. However, six thousand years ago the Andals invaded Westeros, conquering the First Men and killing the Children of the Forest wherever they encountered them.