Answer:
While the United States was still struggling to emerge from the Great Depression at the end of the 1930s, and would do so partly because of the war, Japan had emerged from its own period of depression, which had begun in 1926, by the mid-1930s. Many of the young soldiers mobilized into the Japanese army by the early 1930s came from the rural areas, where the effects of the depression were devastating and poverty was widespread. Their commitment to the military effort to expand Japanese territory to achieve economic security can be understood partly in these terms. The depression ended in the mid-1930s in Japan partly because of government deficits used to expand greatly both heavy industry and the military.
Internationally, this was a time when "free trade" was in disrepute. The great powers not only jealously protected their special economic rights within their colonies and spheres of influence, but sought to bolster their sagging economies through high tariffs, dumping of goods, and other trade manipulation. The Japanese, with few natural resources, sought to copy this pattern. They used cutthroat trade practices to sell textiles and other light industrial goods in the East Asian and U.S. markets, severely undercutting British and European manufacturers. They also developed sources of raw materials and heavy industry in the colonies they established in Korea, Taiwan and Manchuria. Japan used high tariffs to limit imports of American and European industrial products.
The Japanese military faced a particular tactical problem in that certain critical raw materials — especially oil and rubber — were not available within the Japanese sphere of influence. Instead, Japan received most of its oil from the United States and rubber from British Malaya, the very two Western nations trying to restrict Japan's expansion. U.S. President Franklin D. Roosevelt's embargo of oil exports to Japan pressured the Japanese navy, which had stocks for only about six months of operations.
The Japanese army, for its part, was originally concerned with fighting the Soviet Union, because of the army's preoccupation with Manchuria and China. The Japanese army governed Manchuria indirectly through the "puppet" state of Manchukuo and developed heavy industry there under its favorite agencies, disliking and distrusting the zaibatsu (large Japanese corporations). But the Soviet army's resistance to Japanese attacks was sufficient to discourage northern expansion.
Meanwhile in 1937, the intensification of Chinese resistance to the pressure of the Japanese military drew Japan into a draining war in the vast reaches of China proper, and in 1940 into operations in French Indochina, far to the south. Thus, when the navy pressed for a "southern" strategy of attacking Dutch Indonesia to get its oil and British Malaya to control its rubber, the army agreed.
While it seems that economic factors were important in Japanese expansion in East Asia, it would be too much to say that colonialism, trade protection, and the American embargo compelled Japan to take this course. Domestic politics, ideology and racism also played a role.
Domestic Politics
The political structure of Japan at this time was inherited from the Meiji era and was increasingly dominated by the military. During the Meiji period, the government was controlled by a small ruling group of elder statesmen who had overthrown the shogun and established the new centralized Japanese state. These men used their position to coordinate the bureaucracy, the military, the parliament, the Imperial Household, and other branches of government. Following their deaths in the early 1920s, no single governmental institution was able to establish full control, until the 1931 Manchurian Incident, when Japan took control of Manchuria. This began a process in which the military behaved autonomously on the Asian mainland and with increasing authority in politics at home.
