"Passionate," "intense," "tough," and above all, "ruthless." These were some of the words used to describe Robert Kennedy.

Kennedy's tough-guy reputation emerged from a career of battles. He waged moral crusades against dangers including foreign enemies (communism, Fidel Castro, the U.S.S.R.); domestic adversaries (corrupt unions, the Mafia); specific people (Jimmy Hoffa, "Big Steel" bosses); and abstract concepts (racism, poverty). One of his biggest enemies, Lyndon Johnson, was technically on his side, a fellow Democrat.

"Robert Kennedy, particularly early in his career, tended to see things in blacks and whites," says historian Jeff Shesol. "You were either in his favor or you were entirely out of his favor."

Learn about Robert Kennedy's conflicts with four adversaries:

Communism

Fidel Castro

Jimmy Hoffa

Lyndon B. Johnson