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 United States, 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.