Why did the United States and the South Korean struggle at the start of the war?

The Korean War was the first major conflict following the end of World War II and the first war of the Cold War between the United States and Soviet Union. The Korean War was fought between North Koreans and the South Koreans throughout the early part of the 1950s. The north had the support of communist allies including the Soviet Union and China, while the south had the support of the west with the United States. In fact, the United States would play a large role in the conflict for several years. Due to the time period and nature of the Korean War historians today consider it to be a Cold War era proxy war between the United States and Soviet Union.

The main reason the United States got involved in Korea was the purpose of doing everything possible to keep communism from spreading around world. Since the beginning of the Cold War the United States had practiced a policy of containment against the expansion of communism, and wanted to prevent the ideology from taking root in different regions of the world.  This containment policy is often referred to as the Truman Doctrine, since American President Harry S. Truman argued that the United States should actively support the containment of Soviet Communism in the years immediately after World War II.  Further to this idea, the reasons for American involvement in the Korean War are often held as part of the Domino Theory.

Explanation:

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d) all choices are correct

the indians crossed artificial boundaries, setting the stage for conflict