How could there be Japanese troops in the northeast before September 18th?

Probably because of the Russo-Japanese War, the Russians were defeated. Of course, they want to cede Russia, the traditional sphere of influence in northeast China, to Japan, and China has been weak for a long time, so of course he can't drive away the Japanese people. Later, it was known that he colluded with the Japanese, but did not encroach on his interests, so he was killed by the Japanese. Because he colluded with the Japanese,