In the near future, three armies have developed to the point of controlling Earth, and fight to expand their borders.
It all starts when China attacks Russia, and creates the Great Empire. It starts conquering Asian lands, and Australia in a relative world indifference. A new energy source is discovered, that lets the economy boom again, but it proves itself a huge threat (it's basically a new atom, that is a stable isotope of a really unstable one), and to prevent the Great Empire from accessing too much of it, an Alliance is formed between Europe and the US, and the 3rd World War starts.
After 40 years of war, without a winner, a group of pacifists exile themselves on the Moon, and become the Mooners. They then enter the War to try and protect what remains of Earth, and that's when the story of players start, by enrolling in one of these three factions...