After the Korean War, the People's Republic of China decided to attempt an invasion on the newly unified Korea, in an attempt to gain more land. This started the Sino-Korean War. Despite China's power, help from various powerful countries like Japan and the United States managed to fend off the Chinese forces. Due to the war, Korea also ended up gaining part of China's land. In 1962, Mao Zedong and Nikita Khrushchev agreed to fuse China and the Soviet Union together, creating the Sino-Soviet Republics, a Communist superpower. Not much happened after that, until 1965, when the Sino-Soviet Republics and Cuba created the SSR-Cuban Defense Agreement; allowing Cuba and the SSR to aid each other if they get into any conflicts. In 1978, major democratic protests occurred around the SSR. The protesters ended up arrested due to their democratic support. This caused major outrage and led to the Sino-Soviet Civil War. With help from the UK, US, Korea, and Japan, the democrats won. Due to the SSR becoming democratic, it falls and splits into numerous countries. The Sino Republic broke into China, Guangdong, Heliongjiang, Jiangxi, Tibet, and Taiwan. Inner Mongolia and Hainan also seceded as well, but were annexed to Mongolia and Japan respectively.
The People's Republic of China had a very good economy, having a greater population than any other country and a giant military. After it fused with the Soviet Union, these numbers went up even higher, and together they ultimately became the most powerful country in the world at the time.