The Sandinista National Liberation Front (Frente Sandinista de Liberación Nacional--FSLN) was formally organized in Nicaragua in 1961. Founded by José Carlos Fonseca Amador, Silvio Mayorga, and Tomás Borge Martínez, the FSLN began in the late 1950s as a group of student activists at the National Autonomous University of Nicaragua (Universidad Nacional Autónoma de Nicaragua--UNAN) in Managua. Many of the early members were imprisoned. Borge spent several years in jail, and Fonseca spent several years in exile in Mexico, Cuba, and Costa Rica. Beginning with approximately twenty members in the early 1960s, the FSLN continued to struggle and grow in numbers. By the early 1970s, the group had gained enough support from peasants and students groups to launch limited military initiatives.

On December 27, 1974, a group of FSLN guerrillas seized the home of a former government official and took as hostages a handful of leading Nicaraguan officials, many of whom were Somoza relatives. With the mediation of Archbishop Obando y Bravo, the government and the guerrillas reached an agreement on December 30 that humiliated and further debilitated the Somoza regime. The guerrillas received US$1 million ransom, had a government declaration read over the radio and printed in La Prensa, and succeeded in getting fourteen Sandinista prisoners released from jail and flown to Cuba along with the kidnappers. The guerrilla movement's prestige soared because of this successful operation. The act also established the FSLN strategy of revolution as an effective alternative to Udel's policy of promoting change peacefully. The Somoza government responded to the increased opposition with further censorship, intimidation, torture, and murder.

In 1975 Anastasio Somoza Debayle and the National Guard launched another violent and repressive campaign against the FSLN. The government imposed a state of siege, censoring the press, and threatening all opponents with detention and torture. The National Guard increased its violence against individuals and communities suspected of collaborating with the Sandinistas. In less than a year, it killed many of the FSLN guerrillas, including one of its founders, José Carlos Fonseca Amador. The rampant violation of human rights brought national and international condemnation of the Somoza regime and added supporters to the Sandinista cause.

In late 1975, the repressive campaign of the National Guard and the growth of the group caused the FSLN to split into three factions. These three factions--Proletarians (Proletarios), Prolonged Popular War, and the Insurrectional Faction, more popularly known as the Third Way--insisted on different paths to carry out the revolution. The Proletarian faction, headed by Jaime Wheelock Román, followed traditional Marxist thought and sought to organize factory workers and people in poor neighborhoods. The Prolonged War faction, led by Tomás Borge and Henry Ruiz after the death of Fonseca, was influenced by the philosophy of Mao Zedong and believed that a revolution would require a long insurrection that included peasants and labor mo36b movements. The Third Way faction was more pragmatic and called for ideological pluralism. Its members argued that social conditions in Nicaragua were ripe for an immediate insurrection. Led by Daniel José Ortega Saavedra and his brother Humberto Ortega Saavedra, the Third Way faction supported joint efforts with non-Marxist groups to strengthen and accelerate the insurrection movement against Anastasio Somoza Debayle. The FSLN's overall growing success led the factions to gradually coalesce, with the Third Way's political philosophy of pluralism eventually prevailing.

Data as of December 1993