Invasion Of The First Men

Before the invasion of Aegon the Conqueror, Westeros had been subject to two major invasions - the first was the invasion of the First Men and the second was the invasion of the Andals. The Children of the Forest and the Giants lived in Westeros before mankind arrived; a large portion of this period is referred to as the Dawn Age. However, the First Men reached Westeros by crossing the Arm of Dorne, which separated the continent from Essos. This marked the start of a conflict between men and children that lasted for thousands of years before coming to an agreement that also marked the end of the dawning age.

Clearly, historical retellings of British mythology served as inspiration for this. Several stories from British mythology claim that Elves and other mythological creatures lived in the British Isles before there were any people there. Then humans landed in Britain and engaged in a land battle with the magical creatures. This reminds the audience a lot of the Dawn Age story.