The Second Sino-Japanese War, also known as the Second China–Japan War and the Chinese War of Resistance against Japanese Aggression, was fought between the Republic of China and the Empire of Japan from 1937 to 1945, as part of World War II. 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 by the Western allies.
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, the pretext that Japan fabricated to justify their subsequent invasion of Manchuria, to be the beginning of the war. 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. After failing to stop the Japanese in the Battle of Wuhan, the Chinese central government relocated to Chongqing (Chungking) in the Chinese interior. Following the Sino-Soviet Non-Aggression Pact, strong material support bolstered the Republic of China Army and Air Force. By 1939, after Chinese victories in Changsha and Guangxi, and with Japan's lines of communications stretched deep into the Chinese interior, the war reached a stalemate. While Japan could control large cities it captured, it lacked sufficient manpower to control the vast countryside. The Japanese were unable to defeat Chinese Communist Party (CCP) forces in Shaanxi, who waged a campaign of sabotage and guerrilla warfare. However Japan ultimately succeeded in the year-long Battle of South Guangxi, occupying Nanning and cutting off the last sea access to Chongqing. In November 1939, Chinese nationalist forces launched a large scale winter offensive, and in August 1940, CCP forces launched an offensive in central China. In December 1941, Japan launched its surprise attack on Pearl Harbor and declared war on the United States. The US increased its flow of aid to China: the Lend-Lease act gave China a total of $1.6 billion ($20.19 billion 2023). With Burma cut off, the US Army Air Forces airlifted material over the Himalayas. In 1944, Japan launched Operation Ichi-Go, the invasion of Henan and Changsha. However, this failed to bring about the surrender of Chinese forces. In 1945, the Chinese Expeditionary Force resumed its advance in Burma and completed the Ledo Road linking India to China. At the same time, China launched large counteroffensives in South China and repulsed a failed Japanese invasion of West Hunan and recaptured Japanese occupied regions of Guangxi.
Japan formally surrendered on 2 September 1945, following the atomic bombings of Hiroshima and Nagasaki along with Soviet offensives recently launched at Japanese forces. China was recognized as one of the Big Four Allies during the war, regained all territories lost to Japan, and became one of the five permanent members of the United Nations Security Council. The Chinese Civil War resumed in 1946, with the Chinese Communist Party consequently gaining the upper hand with help from the Soviets, and ultimately ending with a communist victory, which established the People's Republic of China.