The Second Sino-Japanese War (July 7, 1937 – September 9, 1945) was a military conflict fought primarily between the Republic of China and the Empire of Japan from 1937 to 1945. It followed the First Sino-Japanese War of 1894–95.

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.

 It accounted for the majority of civilian and military casualties in the Pacific War, with anywhere between 10 and 25 million Chinese civilians and over 4 million Chinese and Japanese military personnel dying from war-related violence, famine, and other causes