December 1941: Japanese invasion of Southeast Asia

Alongside its attack on the US naval base at Pearl Harbor, Japan initiated an invasion of the Philippines, Burma, Malaysia, Thailand, and the East Indies in December 1941. This campaign was successful in capturing the East Indies, Philippines, and Malaysia, while Thailand joined the Axis. Fighting continued in Burma for the remainder of the war, although Japan was successful in taking over the main arteries of the country by May 1942. Japan remained in possession of Southeast Asia, with the exception of its loss of Burma in late 1944, until the time of its surrender in August 1945.

The primary motivation for the expansion of the Second World War in Asia was Japan’s own resource shortages. The USA declared an oil embargo against Japan in August 1941, depriving them of their main source of fuel. Japan needed another source of oil to continue its war against China and the only available option was the oil fields in the Netherlandish East Indies. Japan intended to invade the East Indies and, thus, seize control of its oil supply. In doing so, Japan would have invaded an Allied nation -- Netherlands -- and would almost certainly trigger war with Britain and France. Thus, Japan intended to simultaneously strike against all European colonial possessions in Southeast Asia. The Philippines, an American colony, was attacked for a similar reason, that it posed a security threat now that Japan had declared war on the USA.

Seizing the American and European colonial possessions in Southeast Asia both provided Japan with badly needed raw materials and deprived the Allies of naval and air bases from which they could potentially threaten Japan. Of particular importance were Manila, Hong Kong, and Singapore; the capture of these ports depleted the capacity of any outside power to operate in Southeast Asia. Their capture thus guaranteed Japanese maritime and air supremacy in Southeast Asia, a situation that persisted until the end of the war. In the case of some colonies, particularly Malaysia and the East Indies, they also provided resources important to war industry, like rubber and tin.

Another motivation in the Japanese invasion of Southeast Asia was expanding the blockade against the Guomindang. Following the capture of all major Chinese ports by 1939 and the closure of the Kunming–Haiphong railway in 1940, the Guomindang had become dependent on access through the Burma Road to supplement military aid from the USSR. By invading Burma, the Japanese were able to cut this connection and leave the Guomindang with only Soviet aid and whatever American aid could be flown over the Himalayas. Japanese maritime dominance in Southeast Asia also strengthened the effectiveness of the existing blockade around southern China.

The invasion of Southeast Asia was a massive boon to Japan by giving it the resources without which it would have been forced to stop fighting. Japan managed to continue fighting the Second World War because of its access to the resources of Malaysia and the East Indies. Similarly, those resources were deprived from the European Allies, forcing them to depend even more heavily on the USA for supplies. The invasion also directly connects the war in Europe and the war in Asia for the first time, as now Japan is also at war with the European Allies, rather than just China or the USA.