The New Fourth Army (新四軍 Pinyin: xin-si-jun) and the Eighth Route Army were the two main communist forces from 1938. The New Fourth Army was active south of the Yangtze River (Chang Jiang), while the Eighth Route Army was based in Yan'an in the northwest.

Members of the New Fourth Army wore their badges on the left arm, with N4A and the soldier's unit and name listed on the badge.

Immediate Background: After the Xi'an Incident, the Kuomintang led by Chiang Kai-shek and the Communist Party of China led by Mao Zedong formed a United Front against Japan, which was already in control of Manchuria. This was the second of the KMT-CPC United Fronts. The Marco Polo Bridge Incident in July 1937 marked the beginning of the Sino-Japanese War (1937-1945).

In October, 1937, an announcement was made that red army soldiers active in the eight provinces in southern China (those who did not embark on the long march) would be part of the New Fourth Army. The New Fourth Army was established on Christmas Day, 1937 in Hankou, moving to Nanchang on January 6, 1938, when the detachments began marching to the battlefront. At the beginning, the New Fourth Army had four detachments and one task force battalion and numbered roughly ten thousand. Later the army moved to Anhui province. Ye Ting was the army commander, Xiang Ying the deputy army commander.

It was in theory a united front against Japan but in practice there was friction between Nationalist and Communist Forces, which intensified in the fall of 1940. See New Fourth Army Incident.