On October 14, 1962, U.S. reconnaissance planes discovered Soviet nuclear missile sites being constructed in Cuba, marking the start of the Cuban Missile Crisis. Tensions escalated between the U.S. and the Soviet Union as both sides prepared for the possibility of nuclear war. President John F. Kennedy ordered a naval blockade of Cuba, demanding the removal of the missiles. After 13 tense days, Soviet leader Nikita Khrushchev agreed to dismantle the missile sites in exchange for a U.S. promise not to invade Cuba. The crisis marked one of the closest moments the world came to nuclear war during the Cold War.