“Once, dragons and men coexisted. They shared a peace forged in wisdom, a peace that lasted many generations. All that was lost when mankind disrupted this balance in a sudden onslaught. Man fought dragon is a savage war that shook the foundations of their world. This war was called The Scouring. Defeated and humbled, dragons vanished from the realm. In time, man rebuilt and spread his dominion across the land and on the islands beyond. A millennium has passed since those dark days ended.”
—The Opening of Fire Emblem: Rekka no Ken
The Scouring is the name given to the great war between humans and Dragons, a thousand years before the events of Fire Emblem: Binding Blade. Its cause was further explained in Fire Emblem: Rekka no Ken.
Before the war, mankind and the dragonkin lived in peace until humanity declared war on the dragons. In order to gain the upper hand against the numerous humans, the dragons requested the aid of the Divine Dragons, but when they refused to take part, the dragons captured the Divine Dragon girl Idenn and transformed her into the Demon Dragon, destroying her soul and giving her the ability to spawn War Dragons. Around this time a series of elemental imbalances started to occur in the world and these imbalances were blamed on the dragons, although Jahn claims that it was actually the tremendous power of Divine Weapons wielded by the Eight Heroes which had caused it. Due to the fact that these imbalances forced the dragons to seal their power into Dragonstones, the war ended with the victory of the humans and the retreat of the dragons through the Dragon's Gate.