avoid closing in on the China border at the Yalu river
The Korean War was a proxy war between the Chinese and Americans.
The U.N
Chiang Kai-shek.
Japan began its invasion of China in 1931. The Japanese military had their way over the armed forces of China, and occupied parts of China important to their war aims.
It could have discontinued Russian involvement in World War 1.
The Vietnam War might have been avoided, if communism had not been involved.
By diplomacy
No UN forces entered China. UN Forces stopped short of the Chinese border.
Red Chinese Forces were in Korea. US/Allied Forces did not fight in China. Had US/Allied Forces entered Red China, the Soviet Union may have then entered the war. Joseph Stalin still led the Soviet Union at that time and he was no stranger to war; he had just defeated Hitler's Germany, with allied help...
The Korean War was a proxy war between the Chinese and Americans.
They were afraid that the advancing UN forces would not stop at the border but follow the fleeing North Korean forces into China, thus invading China.
In 1928, China was in the midst of a civil war between the pro-democracy Nationalist forces and the Communist forces. It was not until 1949 that Mainland China was united under one government with the victory of the Communist Forces and the flight of the Nationalist forces to Taiwan.
No, it was not avoided. It occurred.
China originally formed a shadow force in the Korean War, supplying North Korean forces with equipment and training. However, as UN forces continued the campaign against the North Koreans, China ultimately entered the war openly, committing troops and resources to the fight, allying with the North Koreans against the UN forces deployed from the south.
The U.N
Traditionalists against radical reformers.
You might have prevented World War I by protecting Rasputin from the attempt on his life. You might also have tried to keep Caillaux in power.