themaesters:

c.-12,000 - The First Men invade Westeros: A human ethnic group from Essos, the First Men, invades Westeros by crossing the Arm of Dorne, bearing weapons of bronze. In a futile attempt to end the invasion, the children use the hammer of the waters to shatter the land bridge, creating the Broken Arm and the island chain known as the Stepstones.

The First Men are more numerous, larger, stronger, and more technologically advanced than the children, who try to resist the invaders using their magic and obsidian weapons. It proves unsuccessful, however, and the First Men gradually push deeper and deeper into Westeros, establishing hundreds of petty kingdoms.

ASOIAF + The Timeline of Major Events