The answer is: the attack on the Moncada barracks.
Fidel Castro launched a failed attack on the Moncada Barracks in 1953 to overthrow Cuba´s president, Batista, which led to Castro´s imprisonment for a year. After the attack, many leaders were exiled, which was not the case of Fidel Castro, who was imprisoned. However, after his year´s imprisonment he traveled to Mexico, on what could be considered a self-imposed exile given the political conditions in Cuba at that time. From Mexico he gathered a group of followers that formed the Cuban revolutionary movement which eventually would overthrow the government of Batista