The Second Sino-Japanese War ( 1937 – 1945 ) , a theater of World War II , forced an uneasy alliance between the Kuomintang and the PLA . Japanese forces committed numerous war atrocities against the civilian population ; in all , as many as 20 million Chinese civilians died . An estimated 40,000 to 300,000 Chinese were massacred in the city of Nanjing alone during the Japanese occupation . During the war , China , along with the UK , the US , and the Soviet Union , were referred to as " trusteeship of the powerful " and were recognized as the Allied " Big Four " in the Declaration by United Nations . Along with the other three great powers , China was one of the four major Allies of World War II , and was later considered one of the primary victors in the war . After the surrender of Japan in 1945 , Taiwan , including the Pescadores , was returned to Chinese control . China emerged victorious but war-ravaged and financially drained . The continued distrust between the Kuomintang and the Communists led to the resumption of civil war . Constitutional rule was established in 1947 , but because of the ongoing unrest , many provisions of the ROC constitution were never implemented in mainland China
war atrocities
Japanese war crimes
China