The United States vowed to contain Soviet expansionism. It demanded and obtained a full Soviet withdrawal from Iran. It supported Turkey against Soviet attempts to control shipping lanes. It provided economic and military aid to Greece to fight a strong Communist insurgency. And it led the effort to airlift millions of tons of supplies to Berlin when the Soviet Union blockaded that divided city.

With most American aid moving across the Atlantic, little could be done to prevent the Communist forces of Mao Zedong from taking control of China in 1949. When North Korea — supported by China and the Soviet Union — invaded South Korea the next year, the United States secured U.N. support for military intervention. The North Koreans were eventually pushed back, and a truce was signed, but tensions would remain high and U.S. troops would stay for decades.

In the mid-1960s, the United States sent troops to defend South Vietnam against a Communist insurgency based in North Vietnam. American involvement escalated greatly but was not enough to prevent the South from collapsing in 1975. The war cost hundreds of thousands of lives. It also caused bitter divisions at home, making Americans wary of further foreign entanglements.