The exact chronology of the repression is still debated, yet the roots of the long political war may have started in 1969 when trade unionists were targeted for assassination by Peronist and Marxist paramilitaries. Individual cases of state-sponsored terrorism against Peronism and the left can be traced back even further to the Bombing of Plaza de Mayo in 1955. The Trelew massacre of 1972, the actions of the Argentine Anticommunist Alliance commencing in 1973, and Isabel Martínez de Perón's "annihilation decrees" against left-wing guerrillas during ''Operativo Independencia'' (Operation Independence) in 1975, are also possible events signaling the beginning of the Dirty War.