Aliens called BETA (Beings of the Extra Terrestrial origin which is Adversary of human race) have invaded and mankind fights back against the alien aggressors with giant robots called Tactial Surface Fighter or TSFs, as almost all conventional weapons before simply didn't work.
The BETA first arrived on Earth in 1973 (in China and Canada) after arriving on the Moon in 1967 and being spotted on Mars in 1958. By 2001, the world population is down to 1 billion people, and most of Europe and central Asia have been wiped out. The Soviet Union, though, is still around, operating out of Alaska. When the BETA first arrived to Japan in 1998, 36 million people died within a week. Numbers of deaths continue to raise as the BETA invades the rest of the world.