New Fourth Army
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, culminating in the New Fourth Army Incident with a full-fledged battle between the New Fourth Army and KMT National Revolutionary Army forces. Up until that point, most of the battles had been skirmishes. The army was fully reorganised after the incident and remained in active combat until the end of the war.