During a televised speech, President John F. Kennedy announced that an american spy plane had discovered Soviet missile bases on the Island nation of Cuba. In 1959, Cuba went under a revaluation and Fiddle Castro took power and as result, Cuba allied itself with the Soviet Union.
The missile silos were close to be finished and could be able to strike the nations capital, Washington D.C. and other major US cities. (History)
The whole world was rocked because if the crisis was botched WW3 would start and many would die.
Kennedy then enforced a blockade to prevent Soviet ships from entering Cuba. Most ships complied to this blockade and altered their route