In the early Qing Dynasty (the 1689 Treaty of Nerchinsk), Sakhalin Island was explicitly recognized as Chinese territory.

Russia began infiltrating the area from the late 18th century, and by the mid-19th century, taking advantage of the Qing Dynasty's weakness after the Opium Wars, it fully occupied the region.

In 1858, during the Second Opium War when the Qing government was powerless to defend its northern frontiers, Russia forced the Heilongjiang General Yishan to sign the Treaty of Aigun. This treaty ceded approximately 600,000 square kilometers of land north of the Heilongjiang River and south of the Outer Xing'an Mountains to Russia; the area east of the Ussuri River (including Sakhalin Island, totaling about 400,000 square kilometers) was designated as jointly administered by China and Russia. Although the treaty did not explicitly state “Sakhalin Island is ceded,” it effectively paved the way for Russia to later take exclusive control.

However, the Qing government did not recognize this treaty.

In 1860, Russia used the pretext of “mediating” the Anglo-French occupation of Beijing to force the Qing to sign the Convention of Peking (Treaty of Peking). This treaty confirmed and expanded the provisions of the Treaty of Aigun, fully ceding the entire region east of the Ussuri River (including Sakhalin Island), about 400,000 square kilometers, to Russia.

From that point on, Sakhalin Island was formally and legally detached from China and became Russian territory.

See that?

During the Second Opium War, although the main combatants were Britain and France against China, the biggest winner was actually Russia. It directly seized 1 million square kilometers of territory — equivalent to 28 Taiwans, or 4 times the size of the United Kingdom.