With the December 1941 Japanese attack on Pearl Harbor, the Second Sino-Japanese War, which had been rumbling on since 1937, was transformed into a major theatre of World War II.
By 1941, the Chinese position was precarious. The largest forces opposing the Japanese were the Nationalists led by Chiang Kai-Shek, but the foreign military aid they had been receiving in the 1930s had dried up because of the war in Europe. Chiang's forces were badly trained, badly disciplined and badly equipped. Their loyalty was questionable. The truce with their Communist rival, the CCP, was fragile. Both sides seemed more intent on maintaining control in their own territory than in fighting the Japanese. Both were expecting and preparing for a fresh civil war as soon as Japan was defeated. Many of Chiang's men also held allegiances to local warlords.
In February 1942, when Congress approved a 500 million dollar loan to China, Roosevelt described China as the US's main ally against Japan. Chiang Kai-Shek was enchanted to now be described as one of the 'Big Four' Allied war-leaders. General 'Vinegar Joe' Stillwell became Chiang's Chief of Staff, as well as commander of US forces in China, Burma and India. Chiang believed China would be the centre of US efforts against Japan.
The reality was different. Difficulties in sending supplies, British reservations, general concern about Chiang's motives, and the urgency of operations in the Pacific and elsewhere meant that China did not become a theatre of main effort for the Allies. Stillwell's mission to improve the efficiency of Chiang's forces and turn the tide against the Japanese proved difficult. Chiang, Stillwell and Chennault disagreed fiercely over how to use the limited aid that could be flown in from India across the 'Hump' (the Himalayan mountains). To the frustration of Chinese Communists and Nationalists, the beginning of Pacific offensives in 1943 meant that US strategy ceased to depend upon China. The priority given to aid for China plummeted.
By 1944, with the air defence situation improving, more supplies began arriving across the Hump. The Ledo Road (later christened the 'Stillwell Road') reopened, having been closed by Japanese conquests in Burma. In April 1944, the 'Ichi-Go' offensive saw the Japanese invade the airfields of Kiangsi and Kwangsi; by June the Peking-Hankow Railway was under Japanese control. Despite US concerns that defeat was looming, Chinese forces resisted, repelling two Japanese offensives during summer 1945. Two events brought the war in China to a swift conclusion: on 6 August, the first atomic bomb was dropped on Hiroshima. Three days later Stalin, honouring his promise to the Western Allies, declared war on Japan, and Soviet forces overran the Japanese army in Manchuria. Japanese forces in China, Formosa and French Indochina surrendered to Chiang. As many as 20 million Chinese had died in the eight year-long conflict. Fighting between the KMT and the CCP resumed almost immediately.