A Return to War.

Following U.S. involvement in two world wars within a twenty-five-year period, many Americans thought they had fought their last war when Japan surrendered in 1945. They were wrong. On 25 June 1950 North Korean Communist troops crossed the 38th parallel into South Korea. Two days later President Truman said that the United States would oppose Communist "armed invasion and war," and he ordered U.S. air and naval forces, under the command of Gen. Douglas MacArthur, to aid in the defense of South Korea.

Rival Blocs.

The United States, however, did not want to become the "world's policeman": that was the job of the United Nations (UN). Formed in 1945, the global organization was founded with the purpose of regulating international conflict and, if need be, of bringing the forces of all the nations of the world against any aggressor...