The Korean War added considerably to the already growing tensions between the United States and the Soviet Union. Over the course of three years, the Korean peninsula became the site of a bitter ideological and military conflict between the respective ideologies of capitalism and communism.
Though not directly involved, the USSR under Stalin supplied the communist North Korean forces with arms, troops, and technical assistance. In the United States, a significant body of opinion held that the Truman Administration, like that of his predecessor FDR, had been soft on communism, standing back and allowing first Eastern Europe, then China to go red. American involvement in the Korean War under the de jure auspices of the UN represented a notable departure from the more hands-off policy previously pursued.
Under the new strategy of containment, the United States pledged itself to resist the spread of communism, wherever it might manifest itself. In turn, this led to a marked deterioration in relations with the Soviet Union, which were fraught enough to begin with. The Soviets accused the United States of pursuing an imperialist agenda, as the Korean peninsula had little or no strategic value for the Americans. The Truman Administration, for its part, recognized the importance of the Korean War as a test of how far the United States was prepared to go in halting the spread of communism. In those terms, the Korean War could be said to be successful from an American standpoint as Stalin's plan to establish communist control over the entire Korean peninsula had been thwarted.
In the longer term, the Korean War set an important precedent that both the United States and the Soviet Union followed consistently throughout the duration of the Cold War—using proxy forces on foreign soil to achieve geopolitical goals.