Sack of Erebor
It began when Smaug the dragon, heard of the great wealth of the Dwarves of Erebor. One night in 2770 TA, Smaug descended on the mountain in flames, totally catching the Dwarves and Men of Dale off guard. The warriors of the mountain came out of the great gate to engage the dragon, but they were all killed. Then Smaug moved and attacked Dale whos warriors were all called to arms, which he destroyed most of them. He then withdrew and routed the mountain of Dwarves. After the all these things he piled up all the gold and made his dwelling only ever so often to eat maidens from Dale which eventually was abandoned.
In the end the Kingdom under the Mountain was destroyed, and likely thousands of Dwarves and several hundred people (including their King Girion of Dale were killed. King Thrór himself along with his son Thráin II barely escaped out of the secret entrance. But many Dwarves escaped mostly fleeing to the Iron Hills.
With his family and a small company of follower, the King went into a homeless wandering southward but not before cursing Smaug. It would be over two hundred years before Thrór's grandson Thorin Oakenshield would return to the mountain to reclaim his home and treasure.