The Second Sino-Japanese War was a military conflict fought primarily between the Republic of China and the Empire of Japan from July 7, 1937 to September 9, 1945. It began with the Marco Polo Bridge Incident in 1937 in which a dispute between Japanese and Chinese troops escalated into a battle.[a] The conflict then escalated further into a full-scale war. It ended with the unconditional surrender of Japan on September 2, 1945 to the Allies of World War II. China fought Japan, with aid to China from the Soviet Union and the United States. After the Japanese attack on Pearl Harbor in 1941, the war merged with other conflicts of World War II as a major sector known as the China Burma India Theater. Some scholars consider the start of the Second Sino-Japanese War in 1937 to have been the beginning of World War II.[23][24] The Second Sino-Japanese War was the largest Asian war in the 20th century.[25] 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 dying from war-related violence, famine, and other causes. 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 polity. Leftists sought universal suffrage and greater rights for workers. Increasing textile production from Chinese mills was adversely affecting Japanese production. The Depression brought about a large slowdown in exports. All of this contributed to militant nationalism, culminating in the rise to power of a militarist fascist 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; some historians, including the PRC government, cite 1931 as the beginning of the war. From 1931–1937, China and Japan continued to skirmish in small, localized engagements, so-called "incidents". Initially the Japanese scored major victories, capturing both Shanghai and the Chinese capital of Nanking in 1937. After failing to stop the Japanese in the Battle of Wuhan, the Chinese central government was relocated to Chongqing (Chungking) in the Chinese interior. 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. The Japanese were also unable to defeat the Chinese communist forces in Shaanxi, which waged a campaign of sabotage and guerrilla warfare against the invaders. While Japan ruled the large cities, they lacked sufficient manpower to control China's vast countryside. On December 7, 1941, the Japanese attacked Pearl Harbor, and the following day the United States declared war on Japan. The United States began to aid China by airlifting material over the Himalayas after the Allied defeat in Burma that closed the Burma Road. In 1944 Japan launched the invasion, Operation Ichi-Go, that conquered 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.