The Second Sino-Japanese War was a military conflict primarily between the Republic of China and the Empire of Japan from 1937 to 1945, as part of World War II. In China, the war is called the War of Resistance against Japanese Aggression (). This total war between China and the Empire of Japan is often regarded as the beginning of World War II in Asia, although some scholars consider the European War and the Pacific War to be entirely separate, albeit concurrent. It was the largest Asian war in the 20th century and has been described as "the Asian Holocaust", in reference to the scale of Imperial Japan's war crimes against Chinese civilians. After the Japanese attacks on Malaya and Pearl Harbor in 1941, the war merged with other conflicts which are generally categorized under the China Burma India Theater of World War II.
When exactly the war began is subject to debate. The conventional start date is 7 July 1937, with the Marco Polo Bridge Incident. Other historians consider the 18 September 1931 Mukden Incident, which led to the Japanese invasion of Manchuria, to be the beginning of the war. The 1931 Mukden Incident was a fabricated pretext for the Japanese invasion of Manchuria. During the invasion, China was aided by the Soviet Union, the United Kingdom, the United States, and Nazi Germany before Germany allied with Japan. Around 20 million people, mostly civilians, were killed. From 1931 to 1937, there were skirmishes between China and Japan in China. But with the Marco Polo Bridge Incident, a dispute between Chinese and Japanese troops in Beijing, the conflict escalated into a full-scale Japanese invasion of the rest of China. Japan achieved major victories, capturing Beijing and Shanghai by 1937. Having fought against each other in the Chinese Civil War since 1927, the Communists and the Nationalists formed the Second United Front in late 1936 to resist the Japanese invasion together.
The Japanese captured the Chinese capital of Nanjing (Nanking) in 1937, which led to the infamous Nanjing Massacre, also known as the Rape of Nanjing.