Why did the united states want to overthrow fidel castro?

At the time of his rise to power and his establishing Communism (or what we call Communism--it was actually Fascism), we and the Soviet Union (which was also in reality a Fascist regime and not a Communist one) were very much at odds with each other. 

That came about because of how World War II played out. Prior to World War II, Europe was the power center of the world. But World War II massacred the economies of Europe, and left them in shambles bad enough to where they could no longer be the power center. 

On the other hand, World War II really revved up the economies of the United States and the Soviet Union. It was clear to everyone near the end of World War II that the United States and the Soviet Union were going to be the major world powers. But both wanted the same areas of influence, mostly Europe, South America, and the Middle East. So America and the Soviet Union were at odds with each other; and entered into the Cold War. 

The Cold War was a military arms race, characterized mostly by a massive increase in military and nuclear technologies. 

Cuba stepped into this war on the side of the Soviet Union, and allowed itself to become a base for Soviet nuclear missiles. Well, Cuba is only 90 miles off the coast of America, so them having nuclear missiles that close presented a grave threat to the United States. 

The attempts to overthrow Fidel Castro were attempts to take Cuba out of the Cold War; being that Cuba's presence was a major asset to the Soviets. But with the collapse of the Soviet Union, Cuba lost most of its importance and Fidel Castro fell to the background. Now he is only brought up when someone in the political arena in America is trying to use it to convince people the opposing side loves Communism. 

Who am I kidding, everyone knows who I'm talking about. Republicans use Fidel Castro as a way to say Democrats are militarily weak...even though they themselves don't do jack about him when they are in power. And that is mostly because Castro has no real power outside of Cuba anymore.