The 20th century was full of foreign policy crises for the United States. After all, this was the century in which the United States participated in two world wars, the Korean War, and the Vietnam War. An argument could be made that many different crises were the most important ever. Many people would probably list World War II as the biggest crisis because of the scope of the war and the sheer evil of Hitler’s regime.
I will argue, however, that the Cuban Missile Crisis was the most important of the 20th century. The reason I say this is because it was the crisis that came the closest to plunging the world into nuclear war. Of course, WWII was terrible and caused human loss on a horrific scale. However, a true all-out war between the Soviet Union and the United States in 1962 would have been immeasurably worse. The two sides would most likely have used nuclear weapons against one another on the battlefield and would likely have used them against each other’s cities. The scale of human loss and of environmental disaster would have been beyond that of WWII by a large degree.
Therefore, though essentially nothing happened, I would argue that the Cuban Missile Crisis was the most important foreign policy crisis of the last century.