Kim met Mao in May 1950. Mao was worried that the US would intervene in the war and gain more economic power, agreed to support the North Korean invasion.
Why did China fight the US in the Korean War?
China desperately needed the economic and military assistance promised by the Soviets. Mao sent Korean PLA veterans and agreed to bring the army closer to the Korean border.
In the Chinese letter, China does not approve of the United States and wants to expel it, as it believes it wants to expand and gain more economic power.
South Korea, on the other hand, viewed the United States as a good country. This is because when North Korea tried to invade South Korea, the United States helped South Korea fight.
Thus, Mao was worried that the US would intervene in the war and gain more economic power, therefore agreed to support the North Korean invasion.
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