How/Why the United States became involved in World War I, World War II and the Korean War?

In June 1950 communist North Korea invaded South Korea. The United States came to the aid of South Korea at the head of a United Nations force composed of more than a dozen countries. Communist China joined North Korea in the war in November 1950, unleashing a massive Chinese ground attack against American forces.

Explanation: The original agreement and intent was to create a unified and independent Korea out of the post Japanese occupation era. Instead each side of the 38th parallel established its own government under the influence of the occupational country; the United States in South Korea and the Soviet Union in North Korea.

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explanation:

north atlantic treaty organization.