In Assassin's Creed III, which historical event does Desmond Miles prevent at the game's conclusion?

In Assassin's Creed III, Desmond Miles prevents the 2012 solar flare at the game's conclusion.

The modern-day storyline builds toward a predicted catastrophe on 21 December 2012, when a powerful solar flare is expected to devastate Earth. Desmond uses the Animus to experience the memories of Connor, whose life provides access to an ancient First Civilization temple and the information needed to reach the crisis point.

At the end of the game, Desmond activates a device associated with the First Civilization. This saves humanity from the solar flare, but it also releases Juno, a powerful member of that civilization, creating a new danger for the future.

The historical storyline is set during the American Revolution and includes figures such as George Washington and Charles Lee. That revolution is not the disaster Desmond prevents; it is the historical setting experienced through Connor's genetic memories.