Why did Japan invade China in 1937?
Japan's invasion of China was due essentially to Japan's desire to be an imperial power. There was both an economic and a militaristic element to this desire.
Economically, Japan needed more resources. It wanted to be an industrial and military power but lacked resources and space on its home islands. For this reason, it expanded into Korea and Taiwan and eventually Manchuria. This was also a reason for wanting to break out of Manchuria and dominate China itself.
Militaristically, Japan was being controlled by hard-line military officers. They wanted war in part simply because part of their ideology extolled war. They wanted to prove that they were strong and could subjugate other countries. It was particularly important to subjugate China, which had traditionally been the power in East Asia.
Economic factors played an important role in driving Japan’s decision to invade Manchuria, which was then a part of China, in 1937. Faced with a lack of resources, Japan badly needed Manchuria, the industrial heartland of China, to produce steel and food for their expanding population. Raw materials required to sustain the Japanese war machine could be easily obtained and extracted from resource-rich China. The Japanese nation was also able to gain much needed heavy industries that had the potential to grow and even expand further.
Japanese military decisions were also heavily influenced by ideas of Pan-Asianism. Despite having undergone rapid industrialisation and Westernisation, Japan was still not recognised as a genuine member of the Western community, but was rather seen as a threat due to its status as an Asian nation (which made it the representative of the Asian powers in the eyes of the West). Increasing frustration with Western disapproval came to shape the changing direction of Japanese self-identification, as well as their changing ideology, and Japan began to move towards the idea of Pan-Asianism. They started to see themselves as a potential leader that could unite all the Asian nations together in a struggle against the Western imperialists. Such reasons seem to legitimise their imperial expansion across China and Asia for they were only seemingly invading the latter to liberate or free them from the exploitative Western colonial powers. In reality, the aggression carried out by the Japanese was driven by an inherent desire to prove themselves superior to the West through the occupation of large swathes of territory and China proved to be a viable starting point for this.
it was due to them wanting to be like USA and RUSSIA :)