"Japan launched aggressive war" is an exaggeration.
During the late 1910s and 1920s, Manchuria was run by the Fangtian Clique. And Japan supported this clique. The Fangtian Clique formed a natural buffer with the Soviet Union. Japan signed up to the 9 power treaty to respect "China"s borders. So what with the Fangian Clique? They fought in the Chinese Civil War during the warlord era, suffered some major defeats after the leader tried to seize Beijieng. The economy went bad under bad economic policy by the leader. The chief economy advisor abandoned the Fangtian Clique. It completely fell apart by 1928. Seeing an opening, the Soviet Union made some attacks on the remnants in order to reestablish control of a Russian built railway that went across Manchuria from the northwest down to the southeast where Vladivostok was. The KMT was busy in the main body of China proper and so lend no assistance at all against the Soviet excursion. Mind you, it was only 4 years early, in 1925, that the Soviet Union set up a puppet communist regime in Mongolia. So while Japan is sitting there, complying with the 9 power treaty to keep the other western powers happy, the Soviet Union is expanding its influence into the area. The Soviet Union wasn't a signatory of the 9 power treaty either by the way. They inserted Chinese communist into China which had to be incorporated into the KMT as part of agreement for getting Soviet aid. A time when the Soviet Comintern was announcing intentions to spread communism all over the world. When all this mentioned, it really doesn't look so surprising that the Japanese would decide to take Manchuria for themselves then. And the invasion of Manchuria was carried out with about only 50,000 troops. Manchuria is a huge area, all taken with just 50,000. Not many casualties on either side really either. And despite the invasion happening 3 years after the total collapse of the Fangtian economy, KMT never moved in. 2 years after the Soviet attack on Manchuria in 1929, no KMT assistance. And again, no KMT assistance when the Japanese invaded. Face it, Manchuria was a sitting duck. Saying all this doesn't go as far as saying Japan was in the right to invade it. But to go as far as saying it was the wrong thing to do as if it was something uniquely "aggressive" in that setting, after all is known, seems kind of exaggerated now doesn't it. With all the colonial activities going on in Indonesia, the Philippines, Vietnam, India, the Middle East, all over Africa, was the Japnese invasion of Manchuria really
that bad? Give me a break. What made it "bad" was that it meant possibly a stronger Japan while the countries that have made it on top of the hill (US, Great Britain, lesser extent France and Russia) didn't want more competition so that is why it gets called "bad" and "aggressive" as if the others did nothing of the sort themselves.
The run up to the start of the second Sino-Japanese war in 1937 was also not so one sided on Japan's fault. The KMT and Chinese communists were busy with their civil war, the great march gets going in 1925, finishes up by Spring 1936, a KMT general makes a secret pact with he Chinese communists in summer 1936 and makes do on that pact by December 1936 in what was called the Xi'an incident in which Chinag Kai-shek's generals kidnapped him and took him before the Chinese communists in order to force Chinag Kai-shek to stop the fight against the Chinese communists under slogans like "Chinese don't fight Chinese" and to instead change focus on starting a war against Japan. Remember, this was December 1936. What Japanese invasion was taking place? There was no major war going on. There was some Japanese activities using collaborators to setup friendly Chinese governance in the border areas around Manchuria. That's it. Chiang Kai-shek was very busy in trying to modernize his forces, getting much advice and equipment from the Germans. He of course did want to get Manchuria when he was ready. But he wanted to finish the communist problem first. So he was forced into stopping the campaign against the communist and started preparing for war against Japan. Does this really sound like an intention for an all out sudden invasion to conquer China by Japan? 7 months later, Chinese and Japanese squabbles happen, the Chinese shoot first at the Marco Polo bridge, thus begins increased hostilities in the northern area. While further south in Shanghai, the Chinese moved their soldiers into Shanghai which was violation of international agreements there. I don't really fault them. So don't get me wrong. The Japanese demanded the Chinese NRA to leave the international area in Shanghai. The Chinese refused. The Chinese then attacked the Japanese positions first and they were the first to go as far as using aircraft to bomb Japanese buildings there, although apparently with some friendly fire bombing. The Chinese strategy was to push the Japanese out soon enough with advantage of surprise to be quick enough about it to prevent enough time for Japanese reinforcements. Japan made the decision to push to Nanjing because they figured the Chinese were not going to stop. Afterall, Chiang Kai-shek wanted to get Manchuria in the end of it all. So total war has been started, so here it goes, the march towards Nanjing. So after all that is put out, was it really entirely a Japanese plan and sudden invasion into China? No not really. What really seemed to have happened is that the corrupt KMT was struggling to finish off the Chinese communists after the long march and the Chinese communist successfully diverted KMT attention to Japan. The Chinese communist didn't help very much against Japan. Just enough to use as propaganda tool for their recruitment campaign as KMT bleeds in doing the bulk of the fighting.
Actually when you think about it.. CKS pushing along together with the US for the total disarmament and dismantling of Imperial Japan was exactly what implanted the US deeply into this region, the very thing that now you pro-CCP posters want to reverse. Funny that