China

China was the main enemy of the United States during the Great War.

The president of China in 2073, and probably until 2077 was called Xin.

It seems that in the 21st century, the Soviet Union lost its position of the dominant communist country to China. Curiously, the only mention of China being communist in any of the Fallout games is in the dialogue with Enclave President Dick Richardson, who calls the Chinese "damn Reds". It also mentioned as such in MCA's Fallout Bible Timeline. It is unknown exactly what kind of political structure the Chinese adhered to. It can be assumed that China as some kind of communist country suits the Fallout setting best, however.

China shows up in Fallout's history notes as the biggest competitor of the United States, not just in military, but likely also economically and scientifically as they attempted to be the first to drill the last oil resource in the world. It is implied that Americans sabotages this attempt and that without such sabotage the Chinese would've been first. The Chinese also try to keep pace on power armor developments, though how much they managed this is unknown.

It is a known fact that China invaded Alaska, though it is disputed in which year (Fallout Bible: 2059 or 2066, Sierra Depot: 2077), but the exact details on China as a nuclear power are sketchy at best. It is not actually known how much nuclear power China brought to the final blasts of the Great War or even if it was the only country (or even one of the countries?) throwing bombs at the United States, who was at that time in dispute with many countries.

The Chinese solution to invasion by U.S. forces in power armor was its Hei Gui stealth armor, worn by elite Chinese "Black Ghost" counterinsurgency/terror units.