The West attempted to supply China through the Burma Road, a 700-mile single-lane road built by hand in 1938. Chiang was difficult to supply, not only because the Japanese repeatedly bombed the Burma Road's terminus in Kunming, but also due to the widespread corruption within the Kuomintang.
When Japan attacked the West in December 1941, additional supplies were already on the way. The Burma Road was lost, forcing the construction of the Ledo Road from India. Meanwhile Chiang was supplied by flights over the Himalayas.
US Army General Joseph Stilwell was given the task of commanding Allied forces in China. Stilwell was highly critical of Chiang Kai Shek's widespread corruption, obsession with the Communists, and lack of emphasis on training. Roosevelt sacked Stilwell at Chiang's urging in 1943 and replaced him with British General Lord Louis Mountbatten.
American war planning at the start of the war favored China as the major area of operations to establish bases that could bomb Japan. Early on, it became clear that due to the political differences and the widespread corruption, China would be a secondary theatre of operations to the island hopping campaigns of the central and south pacific.
On August 8, 1945, the Soviet Union attacked, occupying most of Manchuria by the armistice on August 14th. The Soviets regained the rights they had lost during the first Russo-Japanese War of 1904-05. The Soviets occupied Manchuria until 1949, when the Communists took control of the country.