War breaks out in the Pacific (1941)

[Peter]

Mitsubishi A6M2 "Zero" fighters on the Imperial Japanese Navyaircraft carrier Shōkaku, just before the attack on Pearl Harbor

In 1939 the United States had renounced its trade treaty with Japan and beginning with an aviation gasoline ban in July 1940 Japan had become subject to increasing economic pressure. During this time, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by late September. Despite several offensives by both sides, the war between China and Japan was stalemated by 1940. To increase pressure on China by blocking supply routes, and to better position Japanese forces in the event of a war with the Western powers, Japan had occupied northern Indochina. Afterwards, the United States embargoed iron, steel and mechanical parts against Japan. Other sanctions soon followed.

In August of that year, Chinese communists launched an offensive in Central China; in retaliation, Japan instituted harsh measures in occupied areas to reduce human and material resources for the communists. Continued antipathy between Chinese communist and nationalist forces culminated in armed clashes in January 1941, effectively ending their co-operation. In March, the Japanese 11th army attacked the headquarters of the Chinese 19th army but was repulsed during Battle of Shanggao. In September, Japan attempted to take the city of Changsha again and clashed with Chinese nationalist forces.

German successes in Europe encouraged Japan to increase pressure on European governments in Southeast Asia. The Dutch government agreed to provide Japan some oil supplies from the Dutch East Indies, but negotiations for additional access to their resources ended in failure in June 1941. In July 1941 Japan sent troops to southern Indochina, thus threatening British and Dutch possessions in the Far East. The United States, United Kingdom and other Western governments reacted to this move with a freeze on Japanese assets and a total oil embargo.

Since early 1941 the United States and Japan had been engaged in negotiations in an attempt to improve their strained relations and end the war in China. During these negotiations Japan advanced a number of proposals which were dismissed by the Americans as inadequate. At the same time the US, Britain, and the Netherlands engaged in secret discussions for the joint defence of their territories, in the event of a Japanese attack against any of them.Roosevelt reinforced the Philippines (an American protectorate scheduled for independence in 1946) and warned Japan that the US would react to Japanese attacks against any "neighboring countries".

USS Arizona during the Japanese surprise air attack on the American pacific fleet, 7 December 1941

Frustrated at the lack of progress and feeling the pinch of the American-British-Dutch sanctions, Japan prepared for war. On 20 November it presented an interim proposal as its final offer. It called for the end of American aid to China and the supply of oil and other resources to Japan. In exchange they promised not to launch any attacks in Southeast Asia and to withdraw their forces from their threatening positions in southern Indochina. The American counter-proposal of 26 November required that Japan evacuate all of China without conditions and conclude non-aggression pacts with all Pacific powers. That meant Japan was essentially forced to choose between abandoning its ambitions in China, or seizing the natural resources it needed in the Dutch East Indies by force; the Japanese military did not consider the former an option, and many officers considered the oil embargo an unspoken declaration of war.

USS Arizona during the Japanese surprise air attack on the American pacific fleet, 7 December 1941

Japan planned to rapidly seize European colonies in Asia to create a large defensive perimeter stretching into the Central Pacific; the Japanese would then be free to exploit the resources of Southeast Asia while exhausting the over-stretched Allies by fighting a defensive war. To prevent American intervention while securing the perimeter it was further planned to neutralise the United States Pacific Fleet and the American military presence in the Philippines from the outset.On 7 December (8 December in Asian time zones), 1941, Japan attacked British and American holdings with near-simultaneous offensives against Southeast Asia and the Central Pacific. These included an attack on the American fleet at Pearl Harbor, landings in Thailand and Malaya and the battle of Hong Kong.

These attacks led the United States, Britain, China, Australia and several other states to formally declare war on Japan, whereas the Soviet Union, being heavily involved in large-scale hostilities with European Axis countries, maintained its neutrality agreement with Japan. Germany, followed by the other Axis states, declared war on the United States in solidarity with Japan, citing as justification the American attacks on German submarines and merchant ships that had been ordered by Roosevelt.