What motivated Japan to get into conflict with China? 1931 - take over Manchuria, 1937 - start war with China.
Japan has little or no coal, oil, or industrial mineral resources, and a long history of war with aggressive Siberian and Chinese and Korean governments, often in self defense. Their first military conquests in the 1930s were of major coal deposits in northern China.
"Japan", per se, did not want to get into a war with China. Folks like the Amur River Society (aka The Black Dragon Society) did. The junior officers who provoked the Incident were practicing gekokujo (to push from below), basically setting foreign policy for the nation at the field level.
They wanted Japan to ... well, a bunch of shit. Throw off the yoke of the Occidental Powers, because the arbiter of power in East Asia, achieve the glory due to Yamato, all that. And they thought the government wasn't moving fast enough. Assassination was their means of 'adjusting' the government views. Feb 26, 1936 brought things to a head. https://en.wikipedia.org/wiki/February_26_Incident
China was fighting a civil war before the Marco Polo Bridge Incident. Most of the country was subsistence farmers, with some industry in the cities. Japan was probably the most technologically advanced non-colony in that part of the world. Australia was nearest to Japan, I think.
I wonder if Japan viewed China as a threat. Maybe the Japanese were concerned that if China modernized they may become powerful enough to try to invade Japan.
Did the civil war have anything to do with Communism? I believe that China became a Communist country after World War 2.
Japan had signed the Nine Power Treaty at the Washington Naval Conference, promising to allow equal access to the China markets. That didn't last. Japan wanted an empire and China was to their India.
Other than the failed Mongolian invasions I'm not aware of any other times Japan was invaded. Please clarify how Siberian, Chinese and Korean governments were aggressive towards japan.
The Mongols of the Divine Wind invasions were Siberian and Chinese both. Many of the ships they were using were apparently built by their ally, the Koreans. So that's a famous one. But Korea was in the middle for all those centuries - the obvious conduit for Asian threat (and opportunity) to Japan.
Yes but you are making an assumption. I believe for most of written history, Japan was the aggressor.
In 1592 japan invaded korea and again in 1598. Both invasions were started in order to strike at China. https://en.m.wikipedia.org/wiki/Japanese_invasions_of_Korea_(1592–1598)