Complex sentence: Morazán had the opportunity to shoot Carrera, but did not, because he needed the support of the Guatemalan peasants to counter the attacks of Francisco Ferrera in El Salvador; instead, Morazán left Carrera in charge of a small fort in Mita, without any weapons.
Split into simpler sentences: Morazán had the opportunity to shoot Carrera, but did not, because he needed the support of the Guatemalan peasants to counter the attacks of Francisco Ferrera in El Salvador. Instead, Morazán left Carrera in charge of a small fort in Mita, without any weapons.