The Second Sino-Japanese War (July 7, 1937 to September 9, 1945) was a major war fought between China and Japan, both before and during World War II. It was the largest Asian war in the twentieth century.

Although the two countries had fought intermittently since 1931, full-scale war started in earnest in 1937 and ended only with the surrender of Japan in 1945. The war was the result of a decades-long Japanese imperialist policy aiming to dominate China politically and militarily to secure its vast raw material reserves and other resources. At the same time, the rising tide of Chinese nationalism and notions of self determination stoked the coals of war. Before 1937, China and Japan fought in small, localized engagements in so-called "incidents." Yet the two sides, for a variety of reasons, refrained from fighting a total war. The 1931 invasion of Manchuria by Japan is known as the "Mukden Incident". The last of these incidents was the Marco Polo Bridge Incident of 1937, marking the official beginning of full scale war between the two countries.

From 1937 to 1941, China fought alone. After the Japanese attack on Pearl Harbor, the Second Sino-Japanese War merged into the greater conflict of World War II.