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The short version: Japan’s actions from 1852 to 1945 were motivated by a deep desire to avoid the fate of nineteenth century China and become a great power.
For Japan, the Second World War grew from a conflict historians call “the Second Sino-Japanese War.” The Second Sino-Japanese War began in earnest in 1937 with a battle called “the Marco Polo Bridge Incident.” However, before this, there had been years worth of border clashes between the Japanese and the Chinese, these having started with the 1931 Japanese invasion of Manchuria. So to explain Japan’s behavior in the years from 1941 to 1945, we have to explain why Japan invaded Manchuria in 1931, and in order to do this, we have to go back to 1853.
So, before 1852, Japan was isolationist. Contact with the west was limited to trade with the Dutch in the city of Nagasaki – Westerners otherwise weren’t allowed in the country and western influences were strongly discouraged. In 1853, Commodore Matthew C. Perry of the United States Navy steamed into what we now call Tokyo Bay. The Japanese told him to leave and go to Nagasaki. He ignored the directive and was surrounded by the Japanese fleet. He presented a counter-demand to have a letter from US President Millard Fillmore presented to the de facto ruler of Japan at the time, the Shogun. When this demand was not met, he shelled a few buildings in the harbor. The letter was presented. Perry returned a year later to sign the Convention of Kanagawa, a treaty that opened the Japanese ports of Shimoda (a city between Kyoto and what we now call Tokyo and was then called Edo) and Hakodate (located on the northern island of Hokkaido) to US trade. The terms were dictate by the Americans, the Japanese had little choice but to agree, seeing that they were seriously technologically outmatched.
This is where modern Japanese history begins. The importance of Perry’s missions to Japan in the 1850’s really can’t be overstated. While Japan had previously thought itself to be a strong country, Perry’s actions and the signing of treaties widely viewed in Japan as unequal destroyed this image. While Japan’s isolation had allowed the Japanese to think that they might escape the fate the Chinese were suffering, the end of this isolation gave lie to that idea.
The Japanese were petrified that they’d go the same way China did, and it wasn’t very long before a reform movement got started. In 1868, this reform movement led to what we now call the Meiji Restoration. The Shogun was stripped of his power, which was then nominally placed back in the hands of the Emperor, but really into the hands of his advisers. In a very brief span of time, the feudal system that had governed Japanese society for centuries was abolished, the military was reformed and the country was put on the path to industrialization.
The Japanese knew they had to catch up to the western powers or else risk getting stomped flat by them – which is what had happened to China – so they did a lot of imitation. Western style dress was widely adopted among the elites of the new society, the military was recreated along Clausewitzian lines, the parliament was something of a ripoff of the Prussian one, and so on and so forth.
The thing is, if you’re trying to imitate a nineteenth century European power, you have to engage in imperialism – not engaging in colonialism made a country at the time look weak. In the case of nineteenth century Japan, the obvious target for imperialism was just across the Sea of Japan: Korea. By the 1890’s, Korea was actually seen as a massive liability for Japan: it had not reformed as Japan had, and unlike China, could feasibly be conquered by an interested western nation, which would have given an excellent staging ground for an invasion of Japan. Additionally, the Korean peninsula is rich in iron and coal, which if you’re a rapidly industrializing country in the 19th century, you need. Because Japan is not particularly rich in natural resources, it was particularly advantageous for them to have colonies. Not so advantageous for the colonized, but then again, colonialism isn’t designed for that anyway.
The problem was that Korea was a Chinese tributary state: the Korean king paid tribute to the Chinese emperor. So while the Japanese could, and did, force the Koreans to sign some unequal treaties, the peninsula remained free of the Japanese. However, in 1894, the Chinese sent troops into Korea to help put down a rebellion, not notifying the Japanese before they did so. This was against a previous treaty, so the Japanese also sent in troops. Unsurprisingly, fighting broke out, leading to the 1894-1895 First Sino-Japanese War.