Japan was a small, densely populated nation. Japan wanted to modernize, but needed raw materials, cheap labor, and markets for manufactured goods.
Being an island nation, Japan lacked the space and resources to grow
Prior to its imperialist policy during WW II, Japan had a long history of isolationism.
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When Japan became aggressive in the years leading up to World War II, America followed a policy of isolationism at first. However, after Japan's attack on Pearl Harbor in 1941, the US entered the war and adopted a policy of total war against Japan, seeking to defeat them completely.
Because of the nuclear attack on Hiroshima and Nagasaki. They were like silent bombs so that they can tell the whole world that they have the potential to stand on ground with firm basis of industry, intellect and power.
neocolonialism
Imperialism. Japan is a volcanized island that is made up of many mountains and few resources. They did not have enough land or resources to support their population. This is the main reason that they began to spread their influence beyond their borders, or practice a policy of imperialism.
Western imperialism put Japan in a position where they were excluded from attaining the raw materials they required for economic progress.
America declaring war on Japan and Nazi Germany instead of ignoring European and Asian imperialism.
Japan recognized its similar goals and ideology with respect to nationalism and Imperialism. In addition, the Nazis had specific technological advances that the Japanese wanted to incorporate into their war machine. However, the primary reason was to establish an alliance that would stabilize the war effort for Japan if the Allies took an interest in Japan's plans for the Pacific.
Some people would say it was justified and others would say it wasn't, but Kōtoku Shūsui, a Socialist leader in Japan, vividly described in 1901 imperialism's serious drawbacks (Iriye 1972, 75): "Imperialists in Japan and elsewhere are like drunken men, intoxicated by patriotism and militarism, which are nothing but expressions of their animal instincts. They bleed people white with taxes, expand armaments, divert productive capital for unproductive ends, cause prices to rise, and invite excessive imports. These are all for the sake of the state. Government, education, commerce, and industry are sacrificed to patriotism, which is the root of militarism and imperialism."
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Japan became a modern industrialized power