Second Sino-Japanese War

The Second Sino-Japanese War, known in China as the War of Resistance Against Japanese Aggression, was fought between the Republic of China and the Empire of Japan between 1937 and 1945, following a period of war localized to Manchuria that started in 1931. It is often regarded as the beginning of World War II in Asia, as the wars became heavily intertwined after Japan's entry into World War II. It was the largest Asian war in the 20th century.

On 18 September 1931, the Japanese staged the Mukden incident, a false flag event fabricated to justify their invasion of Manchuria and establishment of the puppet state of Manchukuo. This is sometimes marked as the beginning of the war. From 1931 to 1937, China and Japan engaged in skirmishes, including in Shanghai and in Northern China. Nationalist and Chinese Communist Party (CCP) forces, respectively led by Chiang Kai-shek and Mao Zedong, had fought each other in the Chinese Civil War since 1927. In late 1933, Chiang Kai-shek encircled the Chinese Communists in an attempt to finally destroy them, forcing the Communists into the Long March, resulting in the Communists losing around 90% of their men. As a Japanese invasion became imminent, Chiang eventually formed a United Front with the Communists in 1936, a process that was hastened by the Xi'an Incident.

The war is considered to have begun on 7 July 1937 after the Marco Polo Bridge incident near Beijing, which escalated into full-scale Japanese invasion of the rest of China. Following the protracted Battle of Shanghai, the Japanese captured the capital of Nanjing in 1937 and perpetrated the Nanjing Massacre. After failing to stop the Japanese capture of Wuhan in 1938, China's de facto capital at the time, the Nationalist government relocated to Chongqing in the Chinese interior. After the Sino-Soviet Non-Aggression Pact, Soviet aid bolstered the National Revolutionary Army and Air Force. By 1939, after Chinese victories at Changsha and with Japan's lines of communications stretched deep into the interior, the war reached a stalemate. The Japanese were unable to defeat CCP forces in Shaanxi, who waged a campaign of sabotage and guerrilla warfare. In November 1939, Nationalist forces launched a large scale winter offensive, and in August 1940, CCP forces launched the Hundred Regiments Offensive in central China. In retaliation, Japanese forces implemented massive scorched earth policies in North and Central China, killing millions of civilians.

In April 1941, Soviet aid was halted with the Soviet–Japanese Neutrality Pact. In December 1941, Japan launched a surprise attack on Pearl Harbor and declared war on the United States. The US increased its aid to China under the Lend-Lease Act, becoming its main financial and military supporter. With Burma cut off, the United States Army Air Forces airlifted material over the Himalayas. In 1944, Japan launched Operation Ichi-Go, the invasion of Henan and Changsha. In 1945, the Chinese Expeditionary Force resumed its advance in Burma and completed the Ledo Road linking India to China. China launched large counteroffensives in South China, 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, Soviet declaration of war and subsequent invasion of Manchuria, Inner Mongolia and Korea. The war resulted in the deaths of around 20 million people, mostly Chinese civilians. Japan carried out the largest attacks in the history of biological warfare, causing at least 200,000 deaths. China was recognized as one of the Big Four Allied powers in World War II and one of the "Four Policemen", which formed the foundation of the United Nations. It regained Taiwan and became one of the five permanent members of the United Nations Security Council. The Chinese Civil War resumed in 1946, ending with a communist victory and the Proclamation of the People's Republic of China in 1949, while the government of the Republic of China relocated on Taiwan.