Zhu De and his 1000 remaining troops, who had participated in the abortive Nanchang Uprising, joined Mao Zedong toward the end of April 1928. Together the two joined forces and proclaimed the formation of the Fourth Army. Other veterans who joined the new base included Lin Biao, Zhou Enlai and Chen Yi. The partnership between Mao Zedong and Zhu De marked the heyday of the Jinggang Mountains base area, which rapidly expanded to include, at its peak in the summer of 1928, parts of seven counties with a population of more than 500,000. Together with Yuan Wencai and Wang Zuo's forces, their soldiers numbered more than 8000. A popular story from that period recounts the hardworking Zhu De carrying grain for the troops up the mountain since agriculture was nigh impossible in the mountain range itself. It was also around this period that Mao Zedong formulated his theories of rural-based revolution and guerrilla warfare.