The Japanese wanted to CONTROL China (never really gave well thought out reason for WHY they wanted to, just that they did). One of the things that led to some of the excesses/outright war crimes by the IJA was the frustration creates when the Chinese simply refused to collapse, regardless of what happened on the battlefield.
The Japanese as a state did not really want to control or conquer China, in my estimation. They would have been okay as long as China was not hostile or occupied by hostile powers such as the Soviet Union. This would keep Chinese markets available to Japanese firms.
The Japanese military, especially the Imperial Japanese Army, was a different animal. The IJA was obsessed with a mainland foothold and in particular Manchuria, where battles had been fought in 1895 and 1905. The IJA's continued relevance to Japanese politics hinged on there being some external threat, which China fulfilled quite nicely as both an object of colonial expansion and a burgeoning bogeyman.
These points were hammered into Japanese ultra-nationalist rhetoric in the 1920s and 1930s.
The bogeyman aspect manifested itself varyingly as resurgent Chinese nationalism turning mainland Asia into an anti-Japanese giant, or as the threat of Western or communist powers hijacking the Chinese state to the disadvantage of Japan. The colonial aspect manifested in the supposed need for a "Manchurian lifeline" that would sustain Japanese economic growth in light of a deadlock that came about around the time of the Great Depression.
In sum, it was IJA political ambition riding on economic and geopolitcial fears as well as a good dose of Japanese ultra-nationalism that brought about the outright annexation of Manchuria and by extension the overall war against China as well as the later Pacific War. Everything was rooted in the fact that the Imperial Army and Navy had managed to hijack Tokyo and that not being a coherent decision-making institution, these military factions just tried to one-up each other until they got nuked.
EDIT: The main thing to take away from this is that the Japanese military, not the Japanese state or nation, was primarily responsible for the war. Of course most of the country was to some extent complicit, but that's still a different matter.