and October 1965. These operations were supplemented by increased U.S. military assistance. Beginning in 1965, the U.S. government sent Green Berets and CIA advisers to instruct the Guatemalan military in counterinsurgency (anti-guerrilla warfare). In addition, U.S. police and "Public Safety" advisers were dispatched to reorganize the urban security structures. In a clandestine operation in March 1966, a total of thirty Partido Guatemalteco del Trabajo (PGT, Guatemalan Party of Labor) associates were seized, detained, tortured, and executed by the security forces. When law students at the University of San Carlos used legal measures (such as habeas corpus petitions) to require the government to present the detainees at court, some of the students were "disappeared" in turn. These "disappearances" became notorious as one of the first major instances of mass forced disappearance in Latin American history. The use of this tactic was augmented dramatically after the inauguration of President Julio César Méndez Montenegro, who – in a bid to placate and secure the support of the military establishment – gave it carte blanche to engage in "any means necessary" to pacify the country. With the explicit authorization of the Mendez administration and increased military aid from the United States, the army – accompanied by militarized police units – mounted a large pacification effort in the departments of Zacapa and Izabal in October 1966. This campaign, dubbed "Operation Guatemala," was put under the supervision of Colonel Carlos Arana Osorio, with guidance and training from 1,000 US Green Berets. Under Colonel Arana's jurisdiction and in the city, military strategists armed and fielded various paramilitary death squads to supplement regular army and police units in clandestine terror operations against the FAR's (Rebel Armed Forces, ) civilian support base. Personnel, weapons, funds and operational instructions were supplied to these organizations by the armed forces. The death squads operated with impunity – permitted by the government to kill any civilians deemed to be either insurgents or insurgent collaborators. The paramilitaries or "commissioners" who comprised the clandestine terrorist groups organized by the army were primarily right-wing fanatics with ties to the MLN, founded and led by Mario Sandoval Alarcón, a former participant in the 1954 coup. By 1967, the Guatemalan army claimed to have 1,800 civilian paramilitaries under its direct control. One of the most notorious death squads operating during this period was the MANO, also known as the Mano Blanca ("White Hand"); initially formed by the MLN