The Truman Doctrine is a U.S. foreign policy that pledges American support for U.S.-aligned nations against alleged authoritarian threats. The doctrine originated with the primary goal of countering the growth of the Soviet bloc during the Cold War. It was announced to Congress by President Harry S. Truman on March 12, 1947, and further developed on July 4, 1948, when he pledged to oppose the communist rebellions in Greece and Soviet demands on Turkey. More generally, the Truman Doctrine implied U.S. support for other nations threatened by the Soviet Union. It led to the formation of NATO in 1949. Historians often use Truman's speech to Congress on March 12, 1947, to date the start of the Cold War.