While ideology cannot entirely explain the origins of the cold war, it may help explain why the cold war became so enduring and contentious. Both nations held dramatically different worldviews, nurtured by their domestic values. The Soviet Union envisioned a world-wide global revolution leading to a Communist utopia. The United States believed in democracy and private enterprise. As their World War II coalition melted away in the face of growing political disagreements, the rhetoric of both nations turned shriller and argumentative, making faith in negotiations and treaties virtually non-existent.

In February of 1946, George Kennan, the chargé d'affaires in the American Embassy in Moscow and an expert on Russia, wired the longest telegram in State Department history. The "long telegram," as it became known, expressed doubt about the possibility of direct, armed conflict with the Soviet Union. Kennan believed the greater Soviet threat arose from its support of Communist parties and other subversive elements worldwide. He argued that the strategy to deal with this threat should be to strengthen Western institutions and an American commitment to assist endangered nations. One month later in the small town of Fulton, Missouri, former British Prime Minister Winston S. Churchill spoke of an "iron curtain" having descended across Eastern Europe, creating a "Soviet sphere" of influence just as Kennan warned against.

In July 1946, President Truman asked Clark Clifford, his special counsel, to prepare a report concerning U.S. relations with the Soviet Union. Written with the assistance of George Elsey, a junior naval aide, Clifford's report discussed agreements observed or broken by the Soviet Union and Soviet foreign policy and its effect on the United States. The report also underscored Moscow's determination to expand its military power and obtain the atomic bomb. In direct contrast to Kennan's long telegram, the report stressed the importance of expanding military readiness to deal with the increasing Soviet military threat.

In 1947, Kennan wrote an article for the journal Foreign Affairs entitled "The Sources of Soviet Conduct," published under the pseudonym "X". In the article, Kennan restated many of the points made in his long telegram, and first articulated his idea of containment. Kennan believed the West needed to maintain a steady, firm policy to keep the Soviet threat in check and wait for what Kennan believed was inevitable: the downfall of the Soviet Union. This strategy became the inspiration behind U.S. aid to Greece and Turkey, the Marshall Plan, and the creation of the North Atlantic Treaty Organization (NATO).

Two years after Kennan outlined his ideas about containment, the entire nation was shocked by the detonation of the first Soviet atomic bomb in August 1949. President Truman responded by approving development of the hydrogen bomb and ordering a complete review of U.S. national security policy. The result was National Security Council Paper 68 (NSC 68), approved in April 1950 by President Truman. The report rejected Kennan's view that the Soviets were not militarily prepared to attack the West, and argued that in order for containment to be more than a bluff, the United States must be sufficiently prepared for armed conflict. It recommended a massive military buildup and increased foreign aid.

The Cold War raged on through the remainder of the Truman presidency with occasional flare-ups such as the invasion of South Korea and charges of Communist spies in government. Through the accusation of U.S. spying during the Eisenhower administration, the Cuban missile crisis and the construction of the Berlin Wall in the Kennedy administration, and the alternate arms reductions and increases in the Nixon and Reagan administrations, the Cold War remained active well into the 1980s. The Cold War effectively ended with the fall of the Soviet Union in 1991.