Second Sino-Japanese War
United States

The United States generally avoided taking sides between Japan and China until 1940, providing virtually no aid to China in this period. For instance, the 1934 Silver Purchase Act signed by President Roosevelt caused chaos in China's economy which helped the Japanese war effort. The 1933 Wheat and Cotton Loan mainly benefited American producers, while aiding to a smaller extent both Chinese and Japanese alike. This policy was due to US fear of breaking off profitable trade ties with Japan, in addition to US officials and businesses perception of China as a potential source of massive profit for the US by absorbing surplus American products, as William Appleman Williams states.

From December 1937, events such as the Japanese attack on USS Panay and the Nanjing Massacre swung public opinion in the West sharply against Japan and increased their fear of Japanese expansion, which prompted the United States, the United Kingdom, and France to provide loan assistance for war supply contracts to China. Australia also prevented a Japanese government-owned company from taking over an iron mine in Australia, and banned iron ore exports in 1938. However, in July 1939, negotiations between Japanese Foreign Minister Arita Khatira and the British Ambassador in Tokyo, Robert Craigie, led to an agreement by which the United Kingdom recognized Japanese conquests in China. At the same time, the US government extended a trade agreement with Japan for six months, then fully restored it. Under the agreement, Japan purchased trucks for the Kwantung Army, machine tools for aircraft factories, strategic materials (steel and scrap iron up to 16 October 1940, petrol and petroleum products up to 26 June 1941), and various other much-needed supplies.

In a hearing before the United States Congress House of Representatives Committee on Foreign Affairs on Wednesday, 19 April 1939, the acting chairman Sol Bloom and other Congressmen interviewed Maxwell S. Stewart, a former Foreign Policy Association research staff and economist who charged that America's Neutrality Act and its "neutrality policy" was a massive farce which only benefited Japan and that Japan did not have the capability nor could ever have invaded China without the massive amount of raw material America exported to Japan. America exported far more raw material to Japan than to China in the years 1937–1940. According to the United States Congress, the U.S.'s third largest export destination was Japan until 1940 when France overtook it due to France being at war too. Japan's military machine acquired war materials, automotive equipment, steel, scrap iron, copper, oil, that it wanted from the United States in 1937–1940 and was allowed to purchase aerial bombs, aircraft equipment, and aircraft from America up to the summer of 1938. War essentials exports from the United States to Japan increased by 124% along with a general increase of 41% of all American exports from 1936 to 1937 when Japan invaded China. Japan's war economy was fueled by exports to the United States at over twice the rate immediately preceding the war. According to the U.S. Department of Commerce, Japan corresponded to the following share of American exports;

Japan invaded and occupied the northern part of French Indochina in September 1940 to prevent China from receiving the 10,000 tons of materials delivered monthly by the Allies via the Haiphong–Yunnan Fou Railway line.

On 22 June 1941, Germany attacked the Soviet Union. In spite of non-aggression pacts or trade connections, Hitler's assault threw the world into a frenzy of re-aligning political outlooks and strategic prospects.

On 21 July, Japan occupied the southern part of French Indochina (southern Vietnam and Cambodia), contravening a 1940 gentlemen's agreement not to move into southern French Indochina. From bases in Cambodia and southern Vietnam, Japanese planes could attack Malaya, Singapore, and the Dutch East Indies. As the Japanese occupation of northern French Indochina in 1940 had already cut off supplies from the West to China, the move into southern French Indochina was viewed as a direct threat to British and Dutch colonies. Many principal figures in the Japanese government and military (particularly the navy) were against the move, as they foresaw that it would invite retaliation from the West.

On 24 July 1941, Roosevelt requested Japan withdraw all its forces from Indochina. Two days later the US and the UK began an oil embargo; two days after that the Netherlands joined them. This was a decisive moment in the Second Sino-Japanese War. The loss of oil imports made it impossible for Japan to continue operations in China on a long-term basis. It set the stage for Japan to launch a series of military attacks against the Allies, including the attack on Pearl Harbor on 7 December 1941.

In mid-1941, the United States government financed the creation of the American Volunteer Groups (AVG), of which one the "Flying Tigers" reached China, to replace the withdrawn Soviet volunteers and aircraft. The Flying Tigers did not enter actual combat until after the United States had declared war on Japan. Led by Chennault, their early combat success of 300 kills against a loss of 12 of their newly introduced Curtiss P-40 Warhawk fighters heavily armed with six 0.50-inch caliber machine guns and very fast diving speeds earned them wide recognition at a time when the Chinese Air Force and Allies in the Pacific and SE Asia were suffering heavy losses, and soon afterwards their "boom and zoom" high-speed hit-and-run air combat tactics would be adopted by the United States Army Air Forces.

The Sino-American Cooperative Organization was an organization created by the SACO Treaty signed by the Republic of China and the United States of America in 1942 that established a mutual intelligence gathering entity in China between the respective nations against Japan. It operated in China jointly along with the Office of Strategic Services (OSS), America's first intelligence agency and forerunner of the CIA while also serving as joint training program between the two nations. Among all the wartime missions that Americans set up in China, SACO was the only one that adopted a policy of "total immersion" with the Chinese. The "Rice Paddy Navy" or "What-the-Hell Gang" operated in the China-Burma-India theater, advising and training, forecasting weather and scouting landing areas for USN fleet and Gen Claire Chennault's 14th AF, rescuing downed American flyers, and intercepting Japanese radio traffic. An underlying mission objective during the last year of war was the development and preparation of the China coast for Allied penetration and occupation. Fujian was scouted as a potential staging area and springboard for the future military landing of the Allies of World War II in Japan.
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