He had made many friends amongst the Villistas and Zapatistas at the Convention and was able to convince some of them to depart with him. On December 12, 1914, Carranza issued his Additions to the Plan of Guadalupe, which laid out an ambitious reform program, including Laws of Reform, in conscious imitation of Benito Juárez's Laws of Reform. Once again, Obregón was able to recruit loyal troops by promising them land in return for military service. In this case, in February 1915, the Constitutionalist Army signed an agreement with the "" ("House of the World Worker"), the labor union with anarcho-syndicalist connections which had been established during Francisco I. Madero's presidency. As a result of this agreement, six "Red Battalions" of workers were formed to fight alongside the Constitutionalists against the Conventionists Villa and Zapata. This agreement had the side effect of lending the Carrancistas legitimacy with the urban proletariat. Obregón's forces easily defeated Zapatista forces at Puebla in early 1915. However, the Villistas remained in control of large portions of the country. Forces under Pancho Villa were moving towards the Bajío; Felipe Ángeles' forces occupied Saltillo and thus dominated the northeast; the forces of Calixto Contreras and Rodolfo Fierro controlled western Mexico; and forces under Tomás Urbina were active in Tamaulipas and San Luis Potosí. The armies of Obregón and Villa clashed in four battles, collectively known as the Battle of Celaya, the largest military confrontation in Latin American history before the Falklands War of 1982. The first battle took place on 6 April and 7 April 1915 and ended with the withdrawal of the Villistas. The second, in Celaya, Guanajuato, took place between 13 April and 15 April, when Villa attacked the city of Celaya but was repulsed. The third was the prolonged position battle of Trinidad and Santa Ana del Conde between 29 April and 5 June, which was the definitive battle. Villa was again defeated by Obregón, who lost his right arm in the fight. Villa made a last attempt to stop Obregón's army in Aguascalientes on 10 July but without success. Obregón distinguished himself during the Battle of Celaya by being one of the first Mexicans to comprehend that the introduction of modern field artillery, and especially machine guns, had shifted the battlefield in favor of a defending force.