From A Wiki of Ice and Fire
|Result|| Andals occupy all land south of the Neck.|
First Men hold on to The North
Disappearance of the Children of the Forest
The Andals invasion was a migration of the Andals from Essos to Westeros. This took place between 6 thousand and 4 thousands year before Aegon's Landing. The migration was often violent. The old Kingdoms of the First Men were destroyed and the Children of the Forest were pushed back to the north. Sometimes the take over was more peaceful with intermarriage between Andals and First Men as in the case of House Hightower.
The Andals were the first new invaders after the First Men had settled their peace with the Children of the Forest and lived in harmony with them for 4,000 years. They came from the Hills of Andalos over at Essos. They were tall and fair-haired warriors who carried steel weapons and the seven-pointed star of their gods painted on their bodies. and eventually swept across westeros much as the First Men did thousands of years before.
The Fingers were the places where the Andals first landed, to wrest from the First Men the place that is known today as the Vale, in successive waves they spread. The wars between the First Men and the Andals lasted hundreds of years, but eventually the six southron realms fell to them. Only the Kings of Winter remained in the North. The Andals burned out all the weirwood groves, and slew the children when they found them. believing them abominations, and put the myriad First Men kingdoms of the South to the sword. They created great kingdoms of their own, but this process was somewhat slow, they expanded to the Iron Islands roughly two thousand years after their initial invasion.
Eventually, with Andal kings joining forces and extinguishing of House Mudd (the house of the River and the Hills kingdom of the First Men), the Andals were able to invade the North. However every attack was thrown back by the crannogmen of the Neck or the strong fortifications of Moat Cailin. Eventually the Andals relented and the North was allowed to remain in peace, although over succeeding millennia Andal blood entered the North through dynastic marriages.
One of the major (if largely forgotten) consequences of the invasions is that the Children abandoned Westeros and slowly disappeared over succeeding generations. During and following the Andal invasions, six southron kingdoms of Westeros were forged: the Kingdom of Vale and Sky, the Kingdom of the Rock, the Kingdom of the Reach, the Kingdom of the Iron Islands, the Kingdom of the River and the Kingdom of the Stormlands. At this time Dorne was left as a confederation of bickering, feuding states.
The Andals introduced writing, before that time First Men only used runes for carving on stone, in fact everything written about the Age of Heroes, the Dawn Age, and the Long Night originates from stories written down by septons since. They also introduced the Faith of the Seven to Westeros, in their newly raised kingdoms, where it took root, largely extinguished worship of the Old Gods south of the Neck (a major exception being the Iron Islands, where the native worship of the Drowned God was actually adopted by the invading Andals). They also introduced weapons of iron, the use of horses in warfare and the concept of chivalry.
House Greyiron, which had ruled the Iron Islands for a thousand years, was also defeated during this time.