Sack of Erebor

The Sack of Erebor occurred when the dragon Smaug attacked the Lonely Mountain and drove the Dwarves of Erebor from their capital.

Prelude
After the death of King Dáin I outside his halls at the conclusion of the War of the Dwarves and Dragons, the new King, Thrór, and his brother Grór abandoned the Grey Mountains for safer dwellings with most of their subjects. The king went to the Lonely Mountain (former seat of the Durin's throne), and Grór to the primary source of the Longbeards' iron supply, the Iron Hills. Over the years, the Dwarves of Erebor gained great renown throughout Middle-earth for the splendor of their halls, craftsmanship, and great stores of wealth. Word eventually reached Smaug the dragon, one of the great fire drakes who now dwelled in the Grey Mountains, who in his lust for treasure desired to take the mountain.

The Great Firestorm
One night in TA 2770, Smaug descended upon the mountain in flames, 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 all were killed. Then Smaug attacked Dale whose warriors were all called to arms, and destroyed most of them. He then returned to the mountain and routed the remaining Dwarves. Afterwards he piled up all the gold within the mountain and made it his dwelling, emerging only ever so often to eat maidens from Dale which eventually caused its abandonment.

Aftermath
In the end, Erebor was taken by Smaug and likely thousands of Dwarves and hundreds of Men in the great city of Dale (including Lord Girion) were killed during the terrible firestorm that Smaug committed. King Thrór himself along with his son Thráin II barely escaped out of the secret entrance. Most of the Dwarves who escaped fled to the Iron Hills. With his family and a small company of followers, 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 II Oakenshield would return to the mountain to reclaim his home and treasure. Bard the Bowman, a descendant of Girion, would avenge Girion in slaying Smaug when the dragon attacked Lake-town in TA 2941.

Film adaptation
In the The Hobbit: An Unexpected Journey, the Sack of Erebor appeared in the prologue of the film, with the events narrated by Bilbo Baggins as he wrote his book, There and Back Again.