President Kennedy did not dare invade Cuba, because that action could have started a world war - yet he could not let the missile sites be completed. With his advisers, he decided on a naval blockade to prevent Russian ships delivering the missiles for the Cuban sites.
Khrushchev warned that Russia would see the blockade as an act of war. Russian forces were put on alert; US bombers were put in the air carrying nuclear bombs; preparations were made to invade Cuba. There was massive tension in both Washington and Moscow. Everybody thought the world was going to come to an end. Secretly, the Americans suggested a trade-off of missile bases - US bases in Turkey for Russian bases in Cuba.
The Russians made the first public move. The ships heading for Cuba turned back, and Khrushchev sent a telegram offering to dismantle the Cuban bases if Kennedy lifted the blockade and promised not to invade Cuba. Then, as though having second thoughts, he sent a second letter demanding the dismantling of the Turkish bases. At the vital moment, a US U2 spy plane was shot down.
However, Kennedy ignored the U2 attack and agreed publicly to the first letter, and secretly to the second. The crisis was over.