The Great War (not to be confused with World War I) or World War III, started (and ended) in 2077 on October 23rd. It is not known who dropped the first bomb, but that hardly matters. It lasted a very brief amount of time, but was unbelievably destructive. More energy was released in the early moments of that war than all previous conflicts combined.
The situation leading to the war included China invading Alaska, and the United States annexing Canada. The European Commonwealth dissolved and a world wide conflict followed when each country tried to gather what natural resources it could. This period was known as the Resource Wars.