Why was the United States concerned about nuclear missiles in Cuba?

Fidel Castro had declared that Cuba was communist and that it would align itself with the Soviet union. At this time the cold war was heating up as both the United states and the Soviet union were both competing in a nuclear arms race. When the United states found out that the Soviet union was sending nuclear missiles to Cuba the United states began to worry. Cuba is only 90 miles from the coast of Florida and these missiles were long range and could teach almost anywhere in the United states. President Kennedy ordered a blockade around Cuba and the ship's carrying the missiles turned around avoiding a nuclear war.