Why did the United States enter the Korean War?

Why this is the answer

The one official answer is that the United States entered the Korean War to stop the spread of communism, and the way the war began shows why. After World War II, Korea was split along the 38th parallel. By 1948 a government backed by the Soviet Union ruled the north and a government backed by the United States ruled the south. On June 25, 1950, North Korean troops attacked at several points along the parallel and headed south toward Seoul. The United Nations Security Council condemned the invasion as a "breach of the peace," and the United States and other UN members sent troops to support South Korea. For American leaders this was the policy of the Truman Doctrine put to the test. Three years earlier the country had committed itself to supporting free peoples who resisted outside pressure. The fighting lasted three years and cost the lives of millions of soldiers and civilians. About 1.8 million Americans served in Korea, and 36,576 died there. An armistice signed at Panmunjom on July 27, 1953, stopped the fighting and created a demilitarized zone between the two sides. It was a military cease-fire, not a peace treaty, and North and South Korea are still separate today, with almost the same territory they held when the war began. President Eisenhower, who helped bring about the cease-fire, put it plainly at the time: the country had won an armistice on a single battleground, not peace in the world. The answer for Korea and for Vietnam is word for word the same, which makes the pair easy to study together: two wars in Asia, both fought to stop communism from spreading. The trap is reaching for a more specific cause. North Korea invaded South Korea, not American territory, so "because North Korea attacked the United States" is wrong. Answers about Pearl Harbor or Kuwait belong to other wars.