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 Moscow. 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.

Truman told Congress that "it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures." Truman contended that because totalitarian regimes coerced free peoples, they automatically represented a threat to international peace and the national security of the United States. Truman argued that if Greece and Turkey did not receive the aid, they would inevitably fall out of the United States' sphere of influence and into the communist bloc, with grave consequences throughout the region.

The Truman Doctrine was informally extended to become the basis of American Cold War policy throughout Europe and around the world. It shifted U.S. policy toward the Soviet Union from a wartime alliance to containment of Soviet expansion, as advocated by diplomat George F. Kennan.

At the conclusion of World War II, Turkey was pressured by the Soviet government to allow Soviet shipping to flow freely through the Turkish straits, which connected the Black Sea to the Mediterranean. As the Turkish government would not submit to the Soviet Union's requests, tensions arose in the region, leading to a show of naval force on the site of the Straits. Since British assistance to Turkey had ended in 1947, the U.S. dispatched military aid to ensure that Turkey would retain chief control of the passage. Turkey received $100 million in economic and military aid and the U.S. Navy sent the Midway-class aircraft carrier USS Franklin D. Roosevelt.

In October 1944, British and Greek forces landed in Greece following the gradual withdrawal of Axis occupational forces from the country. Despite the Caserta Agreement stipulating that all Greek resistance factions would join a new Greek Army under British command, General Ronald Scobie ordered the EAM's armed wing, ELAS, to unilaterally disarm on December, 1 1944. EAM responded to the "Scobie Order" by organizing a rally in Athens on December 3 in protest, which was fired upon by Greek security forces, killing 28 protestors. This sparked the Dekemvriana, a series of clashes between EAM and Greek government forces along with their British allies. It ended in EAM's defeat and disarmament under the terms of the Treaty of Varkiza, which marked the end of ELAS and broke EAM's power. This was followed by the White Terror, a period of persecution against Greek leftists, which contributed to the outbreak of the Greek Civil War in 1946.

After the civil war broke out, Communist Party of Greece (KKE) guerrillas revolted against the internationally recognized Greek government which was formed after elections in 1946 which were boycotted by the KKE. The British realized that the KKE were being directly funded by Josip Broz Tito in neighboring Yugoslavia. In line with the Anglo-Soviet percentages agreement, the KKE received no help from the Soviet Union, and Yugoslavia provided them support and sanctuary against Joseph Stalin's wishes. By late 1946, Britain informed the U.S. that due to its own declining economy, it could no longer continue to provide military and economic support to the Greek government.

In 1946–47, the United States and the Soviet Union moved from being wartime allies to Cold War adversaries. The breakdown of Allied cooperation in Germany provided a backdrop of escalating tensions for the Truman Doctrine. To Truman, the growing unrest in Greece began to look like a pincer movement against the oil-rich areas of the Middle East and the warm-water ports of the Mediterranean. In February 1946, George F. Kennan, an American diplomat in Moscow, sent his famed "Long Telegram", which predicted the Soviets would only respond to force and that the best way to handle them would be through a long-term strategy of containment; that is, stopping their geographical expansion. After the British warned that they could no longer help Greece, and following Prime Minister Konstantinos Tsaldaris's visit to Washington, D.C. in December 1946 to ask for assistance, the U.S. State Department formulated a plan. Aid would be given to both Greece and Turkey, to help cool the long-standing rivalry between them.^{[citation needed]}

American policy makers recognized the instability of the region, fearing that if Greece was lost to communism, Turkey would not last long. Similarly, if Turkey yielded to Soviet demands, the position of Greece would be endangered. A regional domino effect threat therefore guided the American decision. Greece and Turkey were strategic allies important for geographical reasons as well, for the fall of Greece would put the Soviets on a particularly dangerous flank for the Turks, and strengthen the Soviet Union's ability to cut off allied supply lines in the event of war.