In 1900, the Boxer Rebellion broke out in China, and Tsarist Russia took the opportunity to send troops to occupy the whole territory of Northeast China, attempting to take it for themselves. This was firmly opposed by the Chinese people and condemned by world public opinion. Japan took the opportunity to form an anti-Russian military alliance with Britain and demanded that Russia withdraw its occupying troops from Northeast China. The negotiations between the two sides were fruitless. Relying on British military support and economic assistance from Britain and the United States, Japan dispatched a navy to surprise the Russian Pacific Fleet anchored outside Lushun Port on February 8, 1904, and sank the Russian warships at Incheon, Korea. Japan and Russia declared war simultaneously on February 10.

After the outbreak of the Russo-Japanese War, Japan actually asked the Qing government to strictly maintain neutrality in the areas outside the three northeastern provinces, leaving the northeastern region as a battlefield and watching Japan and Russia fight in China for the sphere of influence in China. The corrupt Qing government was unable to restrain the warring parties and humiliated declared "outside neutrality".

On September 5, 1905, Japan and Russia signed the "Portsmouth Peace Treaty" in the United States, and without China, they divided the "sphere of influence" in Northeast China without authorization. According to the treaty, Russia ceded to Japan the southern half of Sakhalin Island (south of 50 degrees north latitude), which had occupied our country in the past, and all nearby islands. Russia also recognized Korea as Japan's "protectorate". After the treaty was signed, Japan and Russia immediately forced the Qing government to recognize it. In December 1905, under the pressure of Japan, the Qing government signed the "Treaty on the Matters of the Three Eastern Provinces of the Sino-Japanese Conference" with Japan. In addition to accepting all the provisions of the "Portsmouth Peace Treaty" between Japan and Russia, it also granted Japan certain rights and interests.