The bitter struggle between the KMT and the CPC continued , openly or clandestinely , through the 14-year-long Japanese occupation of various parts of the country ( 1931 – 1945 ) . The two Chinese parties nominally formed a united front to oppose the Japanese in 1937 , during the Second Sino-Japanese War ( 1937 – 1945 ) , which became a part of World War II . Japanese forces committed numerous war atrocities against the civilian population , including biological warfare ( see Unit 731 ) and the Three Alls Policy ( '' Sankō Sakusen '' ) , the three alls being : '' " Kill All , Burn All and Loot All " ''
war atrocities
Japanese war crimes
History of China