Army into a retreat. After a year on the run and having survived numerous attacks, Mao's army eventually arrived at the Shaanxi Soviet, establishing Yan'an as their revolutionary base area. It was here that Mao developed the concept of the "mass line", which attempted to overcome the centralizing and bureaucratic tendencies of Marxism-Leninism by consulting the masses and carrying out the perceived will of the majority. China at war Despite centralizing power over the Yangtze Delta region, the Nationalist government did not hold complete control over China, as a number of regional warlords still remained. In the wake of the assassination of the Fengtian warlord Zhang Zuolin, a power vacuum left behind in Manchuria presented an opportunity for the numerous Korean anarchists that were organized in the region. The Korean People's Association in Manchuria (KPAM) subsequently established an autonomous anarchist zone in Mudanjiang, organized along anarcho-communist lines around the principles of individual freedom and mutual aid. However, the Korean anarchists soon fell victim to attacks by the Empire of Japan, culminating in the Japanese invasion of Manchuria, which dissolved the KPAM and established a puppet state known as Manchukuo. From Hong Kong, Chen Jiongming responded to the invasion by attacking Chiang Kai-shek's regime for its appeasement policy and organizing boycotts of Japanese products. The Empire of Japan continued to conquer more and more regions of China, including Shanghai in 1932 and Rehe in 1933, while also setting up zones of influence throughout North China, establishing puppet states in the Hebei, Chahar and Mengjiang. Despite attempts by volunteers to resist the Japanese incursions in Manchuria and Inner Mongolia, by 1937 the Empire of Japan had launched a full-scale invasion of China, beginning the Second Sino-Japanese War. The Nationalist government had previously been engaging in a number of encirclement campaigns, in an attempt to isolate and destroy the Chinese Soviet Republic, rather than focusing on the threat posed by the Empire of Japan. But following the Xi'an Incident, during which Chiang Kai-shek was detained by two of his subordinates, the Second United Front was established to resist the Japanese invasion. The CCP and KMT fought together in the Battle of Taiyuan and the Battle of Wuhan, losing both to the Empire of Japan, but the two operated largely independently of each other, with the communists favoring guerrilla warfare over conventional battles. In January 1941, a clash between the two parties known as