He notified Americans about the presence of the missiles, explained his decision to enact a naval blockade around Cuba and made it clear the U.S. was prepared to use military force if necessary to neutralize this perceived threat to national security.
This began the 13 days of uncertainty, AKA The Cuban Missile Crisis. The Cubans had the means to start a nuclear war on the U.S if they so pleased, Kennedy responded to this by also threatening to nuke Cuba, should they fire the nukes. This all came to an end when Khrushchev agreed to take the nukes out of Cuba, and Kennedy agreed not to invade Cuba (he also promised to take U.S nukes out of Turkey, as Cuba considered it a threat, but this was not told to the public in order to make Kennedy look better)
