At 7:00 p.m. on October 22, 1962, President Kennedy went on national television and announced a “quarantine” of Cuba — a naval blockade that took effect at 10:00 a.m. Eastern Standard Time on October 24. The ring of U.S. Navy surface ships and aircraft, roughly 800 nautical miles around the island, was empowered to stop, inspect, and turn back any vessel suspected of carrying offensive military equipment.
Kennedy called it a “quarantine” rather than a “blockade” because a blockade was, in international law, an act of war. The word “quarantine” was a softer, medical metaphor that avoided the legal implications of the older term. See the Cuban Missile Crisis.
The U.S. deployed 180 ships, eight aircraft carriers, and substantial submarine and air assets to enforce the line. Twelve Soviet ships were already at sea. Most turned back; a few were stopped; a few drifted until they could be reached by orders from Moscow. The tanker Bucharest reached the line on October 24 and was allowed through. No shots were fired at sea.
The quarantine worked because it bought time. It slowed the flow of matériel, gave the Soviet Union a face-saving off-ramp, and forced a sequence of small, reversible decisions on both sides — the opposite of an air strike, which is one-way. It also gave Kennedy room to act on U-2 imagery showing how close the missile sites were to operational status. The information was incomplete: the CIA and ExComm did not know about the 100 tactical nuclear warheads already in Cuba, including warheads for cruise missiles intended to have sunk the carrier Independence if the invasion had launched. The Organization of American States endorsed the quarantine on October 23 by 19–0, giving the action a regional legitimacy it would not otherwise have had.