The correct answer is "limit the spread of communism."
After World War II, the United States and Soviet Union emerged as global superpowers. These two countries had very different ideas on how a country should be run, with the US favoring a capitalist and democratic system while the Soviet Union favored a communist system. In order to stop the Soviet influence from spreading, the American government adopted the policy of containment. The goal of this policy was to limit the spread of communism.
This is why the US gave hundreds of millions of dollars to Turkey and Greece through the Truman Doctrine. This also explains why the US gave assistance to South Korea and South Vietnam, as both places were in jeopardy of being taken over by a communist government/country.
religious differences and the promise of wealth
The theory is called mercantilism.
Under Kublai Khan the Mongol empire reached its largest extension. He ruled over 24000000 km² (nearly 10000000 mi²). The empire went from China and Korea to Iran and southern Russia, from the Korean peninsula to the Danube River.
During Kublai's reign the empire was divided into four smaller Kanatos but he still remained as the Great Khan of all of them. He was actually the first Khan to successfully conquest China in 1279, making him the first Yuan ruler of the whole China. After his death, the mongols didn't elect a new Khan and the Kanatos became more independent.
