Long ago, some Orlanthi people allied with the dragons, seduced by their kind words and the power they could give them, and formed the mighty Empire of Wyrm's Friends. When the dragons betrayed the Empire and ate their leaders and members, humans from all the surrounding areas decided to march on Dragon Pass, and kill the dragon menace forever. Little did they know that it was a trap, and the dragons feasted on the helpless human armies. It was more of a massacre than a war.
If you want to read more, check out the The Dragonkill War in the game’s Lore screen.