How could it be so easy for the Japanese army to enter Northeast China?

Japan’s aggression against China can be traced back to the Peony Society Incident in 1874. Japan used the Bayao Bay Incident to use force against Taiwan at that time. Afterwards, the Qing government signed the "Beijing Treaty" with Japan. In 1895, the Manchu and Qing government sued for peace and concluded the Treaty of Shimonoseki. According to the treaty: China recognized the independence of Korea, paid an indemnity of 200 million taels of silver, separated the entire island of Taiwan, the Penghu Islands and the Liaodong Peninsula, and opened four commercial ports for trade with Japan. Subsequently, Russia immediately joined forces with Germany, France, etc. to ask Japan to return the Liaodong Peninsula to the Manchu Qing government and increase the compensation by 30 million taels. This was the intervention of the three countries in the return of Liao Dynasty. This sowed hatred between Japan and Russia, leading to the Russo-Japanese War in 1904.

In 1900, the Boxer Rebellion occurred in China, which triggered the Eight-Power Allied Forces Incident. In 1905, in the Russo-Japanese War, Russia was defeated and signed the Treaty of Portsmouth with Japan. Russia recognized Japan's various superior interests in Korea and the right to supervise and protect it; Russia ceded Port Arthur and Dalian to Japan; Russia built Changchun. , the railway between Lushun and its branch lines, and ancillary interests were transferred to Japan. Japan refers to Northeast China and eastern Inner Mongolia as "Manchuria and Mongolia". In addition, Japan annexed the Korean Peninsula through the Japan-Korea Merger Treaty in 1910. Japan has obtained a springboard and forward base for further aggression against China in Northeast Asia.