Answer: US ships had formed a naval quarantine around Cuba.
Step-by-step explanation:
When US reconnaissance found out that the Soviet Union had been installing missiles in Cuba, the government was rightfully terrified because it meant that the Soviets could strike the US before they had time to defend themselves.
This sparked the Cuban Missile crises and led to President Kennedy ordering a naval blockade around Cuba. The Soviets were on their way there with more missiles and the world thought war would finally come if the two Superpowers collided outside Cuba.
Thankfully crises was averted as the leaders of the U.S. and the Soviet Union reached an understanding and the Soviet ships turned back in what was considered a victory for the U.S.