A guerilla foco is a small cadre of revolutionaries operating in a nation's countryside. This guerilla organization was popularized by Che Guevara in his book Guerilla Warfare, which was based on his experiences in the Cuban Revolution. Guevara would go on to argue that a foco was politically necessary for the success of a socialist revolution. Originally Guevara theorized that a foco was only useful in overthrowing personalistic military dictatorships and not liberal democratic capitalism where a peaceful overthrow was believed possible. Years later Guevara would revise his thesis and argue all nations in Latin America, including liberal democracies, could be overthrown by a guerilla foco. Eventually the foco thesis would be that political conditions would not even need to be ripe for revolutions to be successful, since the sheer existence of a guerilla foco would create ripe conditions by itself. Guevara's theory of foco, known as foquismo, was self-described as the application of Marxism-Leninism to Latin American conditions, and would later be further popularized by author Régis Debray. The proposed necessity of a guerilla foco proved influential in Latin America, but was also heavily criticized by other socialists.

Guerrilla Warfare is a military handbook written by Marxist–Leninist revolutionary Che Guevara. Published in 1961 following the Cuban Revolution, it became a reference for thousands of guerrilla fighters in various countries around the world. The book draws upon Guevara's personal experience as a guerrilla soldier during the Cuban Revolution, generalizing for readers who would undertake guerrilla warfare in their own countries.