World War III was started on August 19th, 2090. In response to the establishment of a second United States Colony on Mars, the People's Republic of China and The Russian Federation declared war on the United States, and as a result of treaties, all NATO countries. This war came after the United States suffered it's second Great Depression and defaulted on it's debt for the fifth year in a row.
Tensions between Russia, China and the United States had been mounting for the last nintey years. Finally, the threat of the United States establishing a multi-planetary culture was enough to push the other two super powers to war. It is debated that the economic turmoil in the United States pushed it's enemies to attack, but there were many factors at play.
For the first time since it's founding, the United States fought a war on it's own soil. Chinese and Russian forces landed on both coasts in a coordinated attack on February 2nd, 2092. The fighting was intense, but the greater manpower of China forced the US forces back, allowing the Chinese and Russian troops to establish several beachheads.
Given the size of the United States, the Chinese forces opted for a direct assault on Washington, D.C. in an attempt to cripple the US government. Anticipating this attack, the US government fled to Freedom Colony.
The NATO forces surrendered on July 7th, 2094, with an agreement that the United Nations would be dissolved and replaced with a new, single-world government. The People's Republic of Terra was formed.