The Incursion War was a war that occurred at the end of the 3rd Age, whose end heralded in the beginning of the 4th Age, the time in which the world currently lives.
The aggressors in the war were the Githyanki, a warlike humanoid race from the Astral plane. After manipulating Geryt into war with Bennal and thus weakening both nations, they invaded in force beginning on the eastern side of Norrain, crushing the two nations before a band of legendary heroes infiltrated the caste of their god-queen and killed her, ending their ability to reinforce her army with more forces from their home plane and allowing the remaining Gith to be pushed back and eventually destroyed by the surviving military forces of the material plane.
After the war
Due to the damage done to the world’s military in the process of defeating the githyanki, the formerly powerful nation of Daraska fell apart under the weight of the refugees who had swarmed into their borders; there simply wasn’t enough power to maintain order (which was exacerbated by aristocrats jockeying for political power in such an unstable time) and eventually the nation collapsed into a number of city states and left anything smaller to essentially defend itself.
In the years immediately following the war, scores of villages were destroyed and enslaved due to them being unable to effectively defend themselves, until eventually the aristocrats of the powerful city-states saw the opportunity of mutual benefit: they would defend the towns in exchange for taxes and various trade arrangements (each town being different in its arrangements with their respective lord). And so the world at large slipped into a feudal state of being.