The Second Sino-Japanese War (July 7, 1937 – September 9, 1945), so named due to the First Sino-Japanese War of 1894–95, was a military conflict fought primarily between the Republic of China and the Empire of Japan from 1937 to 1941. China fought Japan, with some economic help from Germany (see Sino-German cooperation until 1941), the Soviet Union and the United States. After the Japanese attack on Pearl Harbor in 1941, the war would merge into the greater conflict of World War II as a major front of what is broadly known as the Pacific War. The Second Sino-Japanese War was the largest Asian war in the 20th century.[9] It also made up more than 90% of the casualties in the Pacific War. (see World War II casualties)

The war was the result of a decades-long Japanese imperialist policy aimed at expanding its influence politically and militarily in order to secure access to raw material reserves and other economic resources in the area, particularly food and labour, and engage war with others in the policy context of aggressive modernized militarism in the Asia-Pacific, at the height of Imperial Rule Assistance Association's Hideki Tojo cabinet and with the order from Emperor Shōwa. Before 1937, China and Japan fought in small, localized engagements, so-called "incidents". In 1931, the Japanese invasion of Manchuria by Japan's Kwantung Army followed the Mukden Incident. The last of these incidents was the Marco Polo Bridge Incident of 1937, which marked the beginning of total war between the two countries.

Initially the Japanese scored major victories, such as the Shanghai, and by the end of 1937 captured the Chinese capital of Nanking. After failing to stop the Japanese in Wuhan, the Chinese central government was relocated to Chongqing in the Chinese interior. By 1939, after Chinese victories in Changsha and Guangxi, and with stretched lines of communications deep into the Chinese interior territories, the war had reached a stalemate. The Japanese were also unable to defeat the

Chinese communist forces in Shaanxi, which continued to perform sabotage operations against the Japanese using guerrilla warfare tactics. On December 7, 1941, the Japanese attacked Pearl Harbor, and the following day (December 8, 1941) the United States declared war on Japan. The United States began to aid China via airlift matériel over the Himalayas after the Allied defeat in Burma that closed the Burma Road. In 1944 Japan launched a massive invasion and conquered Henan and Changsha. However, this failed to bring about the surrender of Chinese forces. Despite continuing to occupy Chinese territory, Japan eventually surrendered on September 2, 1945 to Allied forces following the atomic bombings of Hiroshima and Nagasaki and the Soviet invasion of Japanese-held Manchuria. The remaining Japanese occupation troops in China (excluding Manchuria) formally surrendered on September 9, 1945 with the following International Military Tribunal for the Far East convened on April 29, 1946. At the outcome of the Cairo Conference of November 22–26, 1943, the Allies of World War II decided to restrain and punish the aggression of Japan by restoring all the territories Japan annexed in China, including Manchuria, Formosa, and the Pescadores, to the Republic of China, and to expel Japan from the Korean Peninsula.