The Yellowstone Caldera erupted in 2031. When it did, it caused cataclysmic changes on a global scale.
The caldera is located in the Yellowstone National Park, Wyoming, U.S.A. Its dimensions are 55 kilometres by 72 kilometres.
Due to the vast number of ways such an event could affect the future, there are multiple histories and stories for what "will" happen.
The Famine-Reconstruction History
The United States, after the initial phase of starvation, suicide and conflict, began to rebuild small towns (the large cities were too dangerous). the rest of the world suffered from civil wars, revolutions and famine of biblical proportions. The Second Chinese Revolution, caused by a huge famine, saw millions killed. Japan, unable to grow food, and unable to receive food shipments, became an island graveyard. In Europe, famine and general loss of order led to massive riots, revolutions, and a brutal war over food sources that saw France and Germany bomb each other to dust. Britian, very severely hit by the famine, barely survived the dark years, but never even came close to civil war or revolution.
The World War Three History
When the Yellowstone Supervolcano erupted, it sent the entire planet into chaos. The midwest, or "Breadbasket" of the United States was buried under ash, along with much of the eastern US. Without that food source, millions died of famine and millions more from ash-related deaths (like breathing in too much, or being crushed by ash-overwhelmed roofs). As the United States Government revealed just how helpless it was, people began to riot and fight, each for their own. The US survived in the western portions of the nation, Alaska, Hawaii, California, and the rest of the West Coast States remained intact as a nation. The East Coast, however, was not to lucky. In the Eastern US nobody recognized the US government as a valid entity, with various towns and regions fighting for their own survival. It was absolute chaos, and war spread as the US tried to regain control in those regions. Similar wars were occurring across the planet as people tried to move south, to warmer climates as the entire planet cooled down. Of the 7 billion people that lived on earth when Yellowstone Erupted, only 2 billion survived. The rest died in war, famine, and from the direct affects of the eruption like global cooling and ash fall.