Which action by the United States showed that our policy of isolationism was over?

A. In 1949, the United States joined an alliance of countries (NATO) against the Soviet Union.

B. By joining the Warsaw Pact, it was our major protection against communism.

C. In 1946, we built an Iron Curtain between Eastern and Western Europe.

D. The United States joined the League of Nations to protect ourselves from Communism.