The US spent decades, trying to kill Fidel Castro, and many assume that we still don't have access to the entire story.
The US spent years pushing for regime change in Cuba, most notably in the disastrous Bay of Pigs invasion. And the US focused on Castro specifically. According to Fabian Escalante, the former head of the Cuban Secret Service, there were over 600 attempts to assassinate Castro. These weren't the usual bullet or plane crash scenarios, and the CIA had some unusual help.
The first documented plans to assassinate Fidel Castro, date back to 1960. Escalante claims that every US president, from Eisenhower to Clinton, has at some point attempted to kill Castro. While it's impossible to verify the specifics of Escalante's claims, it is clear that numerous plots existed. It's also clear that many of the plans never moved past the drawing board.
These plans range from the plausible, such as having Castro's mistress poison him, to the outrageous, such as exploding cigars. Many of these assassination attempts involved poison, whether it was to be injected via a hypodermic needle, mixed into a milkshake, or spread inside a wet suit.
But not all of the CIA's alleged plot centered on killing Castro. Plans to dose him with LSD, or remove facial hair, were meant to discredit the leader rather than kill him. While these operations might sound ridiculous today, they were treated with the out most seriousness and secrecy during the Cold War, and, unfortunately, plots to assassinate world leaders are not unusual in the dirty game of international politics.
According to documents released in 2007, the CIA knowingly worked with members of the mafia to assassinate Fidel Castro in the 1960s. Before the Revolution, the mafia had immense power in Havana. When Castro took over, he kicked out the crime families along with foreign corporations. With these enormous financial losses, the criminals certainly had the motivation to see Castro dead.
Through meetings with the mafia member named Johnny Roselli, the CIA eventually partnered with Momo Salvatore Giancana and Santos Trafficante, the head of the Cosa Nostra in Cuba. The group picked a Cuban official named Juan Orta, to carry out the assassination with poison pills. Orta eventually wanted out of the plot, and despite partnering with the organized crime, the CIA was yet again unsuccessful.
The Channel 4 documentary 638 Ways To Kill Castro, covers the mafia's attempts, as well as several more alleged schemes.