War of the First Men and the children of the forest
From A Wiki of Ice and Fire
The war of the First Men and the children of the forest took place during the Dawn Age.
Prelude to War
Prior to the arrival of the First Men on the continent of Westeros, the children of the forest reigned supreme throughout the land. Then, twelve thousand years ago the First Men arrived from Essos across the Arm of Dorne. As the First Men spread throughout the land and built their holdfasts, they cut down trees and burned them. This infuriated the children of the forest, who worshipped trees. They declared war on the First Men.
The two races fought a desperate war for dominance, supposedly resulting in the destruction of the Arm of Dorne and the flooding of the Neck through the magic of the greenseers of the children of the forest. The greenseers are also said to have turned trees into warriors. Despite their obvious advantage in magic, the children were losing. The First Men were equipped with bronze swords and armour, and they were bigger and stronger.
After years of warfare, before they fought one another to a standstill, the two sides agree to a truce, signing the Pact on the Isle of Faces, agreeing to peacefully coexist and granting the open lands to humanity and the forests to the children. In time, the First Men abandoned the gods they had brought from Essos and adopted the gods of the children of the forest. With the signing of the Pact, the Dawn Age ended and the Age of Heroes began. The Pact lasted for four thousand years before the enigmatic Others invaded from the Lands of Always Winter, bringing death and destruction to both races in the Long Night. The Pact between the children and humanity ended with the arrival of new invaders, beginning with the Andals.