The Cold War erupted into a shooting war in June 1950 when troops from Communist North Korea invaded South Korea. For the first time, Americans went into battle to implement containment. Confirming the global reach of the Truman Doctrine, U.S. involvement in Korea also marked the militarization of American foreign policy. The United States, in concert with the United Nations, ultimately held the line in Korea, but at a great cost in lives, dollars, and domestic unity.