Cold War tensions revived in the late 1950s, when both sides began development of intercontinental ballistic missiles (ICBMs). The USSR tried to protect Communist East Germany from a haemorrhage of population to the West by building the Berlin Wall in 1961. Each superpower also attempted to gain influence over emerging nations in Asia, Africa, the Middle East, and Latin America. A serious crisis arose in 1962 when the USSR placed missiles in Cuba, their new ally. President John F. Kennedy threatened nuclear retaliation, and the Soviets withdrew the missiles in return for Kennedy’s promise not to invade Cuba.