Well to put it in simple terms, the Japanese in the 1890s wanted to enter into the club of nations that had empires. As part of their drive to get rid of the Port Treaties imposed on them by the Western powers (US included) they industrialized and decided that empire-building was part of being a modern nation. Following this kind of understanding of the world, the Japanese attacked first the Chinese in 1894 and the Russians in 1905. The motivation behind both these attacks was the colonial acquisition of Korea. By winning both wars Japan was now considered a power in the region and took part in many empire schemes of the West in China. Relations with the West (Russia aside) were quite good at this point.

The Japanese had a treaty with the English which never amounted to much but they fought on the Allied side in WW1, mostly as an opportunity to seize German assets in China. However, with the end of WW1, Western powers were less willing to engage in the competitive style of empire that had led to war while the Japanese, who were comparatively unscathed by the conflict, continued to do so in China. The Japanese Kwantung army in Southern Manchuria, in particular, started acting more and more independent of the government and trying to create a pretext for an invasion of Manchuria. These urges were resisted by the government but with the collapse of the global financial system, the Japanese were desperate for a remedy to their economic problems and acutely aware on their dependence on the international economy. The Kwantung Army provided the answer by staging yet another attack but this time the Japanese government and their people all bought in on it. With the invasion of Manchuria and the installation of a puppet regime there, the Japanese lost a lot of respect from the Wilsonian, League of Nations political climate of the era. This resulted in the Japanese leaving the League of Nations and increased role of the army. The army was now focused on building self-sufficiency and it sought to achieve this through territorial expansion. By the time of their invasion of China in 1937, the Japanese were already widely distrusted by the West. Im running out of battery on my phone so I think I'll stop here for now.