The military opposed Juan Perón's populist government and attempted a coup d'état in 1951 and two in 1955, before succeeding with one later that year. After taking control, the armed forces proscribed Peronism. Soon after the coup, Peronist resistance began organizing in workplaces and trade unions, as the working classes sought economic and social improvements.
Over time, as democratic rule was partially restored but promises of legalizing the expression and political liberties for Peronism were not respected, guerrilla groups began to operate in the 1960s, namely the Peronist Uturuncos and the Guevarist People's Guerrilla Army (EGP). Both were small and quickly defeated.
Jorge Ricardo Masetti, leader of the EGP, which had infiltrated into Salta Province from Bolivia in 1964, is considered by some as Argentina's first "disappeared", as he went missing after the party militants' defeat in clashes with the Argentine gendarmerie.
Prior to 1973 the major revolutionary groups were the Peronist Armed Forces (Fuerzas Armadas Peronistas, FAP), the Marxist-Leninist-Peronist the Revolutionary Armed Forces (Fuerzas Armadas Revolucionarias or FAR), and the Marxist-Leninist Armed Forces of Liberation (Fuerzas Armadas de Liberación or FAL). The FAL guerrillas raided Campo de Mayo in April 1969 and stole 100 assault rifles from the elite 1st Infantry Regiment Patricios.
The extreme left bombed and destroyed numerous buildings in the 1970s in its campaign against the government; these belonged chiefly to military and police hierarchies. But a number of civilian and non-governmental buildings were targeted as well, such as the Sheraton Hotel in Buenos Aires, which was bombed in 1972, killing a woman and injuring her husband; a crowded theatre in downtown Buenos Aires was bombed in 1975.
In 1973, as Juan Perón returned from exile, the Ezeiza massacre marked the end of the alliance between left- and right-wing factions of Peronism. In 1974, Perón withdrew his support of the Montoneros shortly before his death. During the presidency of his widow Isabel, the far-right paramilitary death squad Argentine Anticommunist Alliance (Triple A) emerged.
Armed struggle increased, and in 1975 Isabel signed a number of decrees empowering the military and the police to "annihilate" left-wing subversion, most prominently the People's Revolutionary Army (ERP) armed activity in the province of Tucumán.
Isabel Martínez de Perón was ousted in 1976 by a military coup. According to the International Congress for Victims of Terrorism in 2010, prior to the military takeover in 1976, there were a total of 16,000 casualties (including killed, wounded or abducted) of left-wing terrorism in Argentina, including civilians and military personnel.
Years later in 1995, Argentine intelligence officers claimed that the ERP guerrillas were responsible for the deaths of at least 700 people, in addition to scores of attacks on police and military units, as well as kidnappings and robberies.(wikipedia.org)