Complex sentence: After his release, he continued his clandestine activities in Havana, but when he realized that he was being pursued by the police, decided that it was better to leave for the Eastern Sierra Maestra, and join the guerrilla columns headed by Fidel Castro.
Split into simpler sentences: After his release, he continued his clandestine activities in Havana. Upon discovering that he was under police surveillance, he decided that it was better to leave for the Eastern Sierra Maestra and join the guerrilla columns headed by Fidel Castro.