Cultural Revolution Group. Chen's resistance was the last major open display of opposition to the movement within the PLA. The Gang of Four's Zhang Chunqiao, admitted that the most crucial factor in the Cultural Revolution was not the Red Guards or the Cultural Revolution Group or the "rebel worker" organisations, but the side on which the PLA stood. When the PLA local garrison supported Mao's radicals, they were able to take over the local government successfully, but if they were not cooperative, the seizures of power were unsuccessful. Violent clashes occurred in virtually all cities, according to one historian. In response to the Wuhan Incident, Mao and Jiang Qing began establishing a "workers' armed self-defence force", a "revolutionary armed force of mass character" to counter what he estimated as rightism in "75% of the PLA officer corps." Chongqing city, a center of arms manufacturing, was the site of ferocious armed clashes between the two factions, with one construction site in the city estimated to involve 10,000 combatants with tanks, mobile artillery, anti-aircraft guns and "virtually every kind of conventional weapon." Ten thousand artillery shells were fired in Chongqing during August 1967. Nationwide, a total of 18.77 million firearms, 14,828 artillery pieces, 2,719,545 grenades ended up in civilian hands. They were used in the course of violent struggles ,which mostly took place from 1967 to 1968. In the cities of Chongqing, Xiamen, and Changchun, tanks, armoured vehicles and even warships were deployed in combat. Cleansing the Class Ranks (May–Sept. 1968) In May 1968, Mao launched the massive "Cleansing the Class Ranks" political purge in mainland China. Many were sent to the countryside to work in reeducation camps. On July 27, 1968, the Red Guards' power over the PLA was officially ended, and the establishment government sent in units to besiege areas that remained untouched by the Guards. A year later, the Red Guard factions were dismantled entirely; Mao predicted that the chaos might begin running its own agenda and be tempted to turn against revolutionary ideology. Their purpose had been largely fulfilled; Mao and his radical colleagues had largely overturned establishment power. Liu was expelled from the CCP at the 12th Plenum of the Eighth Central Committee in September 1968, and labelled the "headquarters of the bourgeoisie," seemingly alluding to Mao's Bombard the Headquarters dazibao written two years earlier. Mao's meeting with student Red Guard leaders (July 1968) As the Red