The Second Sino-Japanese War (1937–1945) was a military conflict that was primarily waged between the Republic of China and the Empire of Japan. The war made up the Chinese theater of the wider Pacific Theater of the Second World War. The beginning of the war is conventionally dated to the Marco Polo Bridge Incident on 7 July 1937, when a dispute between Japanese and Chinese troops in Peking escalated into a full-scale invasion. This full-scale war between the Chinese and the Empire of Japan is often regarded as the beginning of World War II in Asia.

China fought Japan with aid from the Soviet Union and the United States. After the Japanese attacks on Malaya and Pearl Harbor in 1941, the war merged with other conflicts which are generally categorized under those conflicts of World War II as a major sector known as the China Burma India Theater. Some scholars consider the European War and the Pacific War to be entirely separate, albeit concurrent, wars. Other scholars consider the start of the full-scale Second Sino-Japanese War in 1937 to have been the beginning of World War II. 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 between 10 and 25 million Chinese civilians and over 4 million Chinese and Japanese military personnel missing or dying from war-related violence, famine, and other causes. The war has been called "the Asian holocaust."The war was the result of a decades-long Japanese imperialist policy to expand its influence politically and militarily in order to secure access to raw material reserves, food, and labor. The period after World War I brought about increasing stress on the Japanese policy. Leftists sought universal suffrage and greater rights for workers. Increasing textile production from Chinese mills was adversely affecting Japanese production and the Great Depression brought about a large slowdown in exports. All of this contributed to militant nationalism, culminating in the rise to power of a militarist faction. This faction was led at its height by the Hideki Tojo cabinet of the Imperial Rule Assistance Association under edict from Emperor Hirohito. In 1931, the Mukden Incident helped spark the Japanese invasion of Manchuria. The Chinese were defeated and Japan created a new puppet state, Manchukuo; many historians cite 1931 as the beginning of the war. From 1931 to 1937, China and Japan continued to skirmish in small, localized engagements, so-called "incidents".

Following the Marco Polo Bridge Incident, the Japanese scored major victories, capturing Beijing, Shanghai and the Chinese capital of Nanjing in 1937, which resulted in the Rape of Nanjing. After failing to stop the Japanese in the Battle of Wuhan, the Chinese central government was relocated to Chongqing (Chungking) in the Chinese interior. Following the Sino-Soviet Treaty of 1937, strong material support helped the Nationalist Army of China and the Chinese Air Force continue to exert strong resistance against the Japanese offensive. 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 the Japanese were also unable to defeat the Chinese communist forces in Shaanxi, who waged a campaign of sabotage and guerrilla warfare against the invaders, they ultimately succeeded in the year-long Battle of South Guangxi to occupy Nanning, which cut off the last sea access to the wartime capital of Chongqing. While Japan ruled the large cities, they lacked sufficient manpower to control China's vast countryside. In November 1939, Chinese nationalist forces launched a large scale winter offensive, while in August 1940, Chinese communist forces launched a counteroffensive in central China. The United States supported China through a series of increasing boycotts against Japan, culminating with cutting off steel and petrol exports into Japan by June 1941. Additionally, American mercenaries such as the Flying Tigers provided extra support to China directly.

In December 1941, Japan launched a surprise attack on Pearl Harbor, and declared war on the United States. The United States declared war in turn and increased its flow of aid to China – with the Lend-Lease act, the United States gave China a total of $1.6 billion ($18.4 billion adjusted for inflation). With Burma cut off it 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 retook West Hunan and Guangxi. Japan formally surrendered on 2 September 1945. China was recognized as one of the Big Four of the Allies during the war, regained all territories lost to Japan and became one of the five permanent members of the United Nations Security Council.