400 years ago that was land nominally subject to the Manchus, but not ruled by them. The locals were left to themselves as long as they occasionally acknowledged Manchu suzerainty, but were expected to cooperate with the Manchus to keep out anyone else. Russians (mostly Cossacks moved in in the 17th century, when the Manchu state was busy conquering China. The locals objected to the Cossacks because they demanded a lot of "tribute" on pain of having your home burned down, death, etc. - i.e. they behaved like bandits. The Manchus sent men to drive them out, but too few - until China was firmly under Manchu control in the 1680s. The Manchus then turned up in force & the Russians wisely moved out, signing the Treaty of Nerchinsk in 1689.Interesting oped on Russia and China: Does China have designs on Siberia? | The Hill
I was not aware that China lost so much territory to Russia as late as 1860. When Russia has been sufficiently weakened, I am guessing China will make a move. A country that is already in conflict with India, is trying to establish the "nine-dash line" in the Sea and also claims Taiwan (that has been independent of China for quite a long time the last few centuries), will most likely reconsider the borders with Russia in the future?
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It's odd that the Chinese claim that territory which was never part of any Chinese-ruled state, but a subject territory of their foreign overlords, is Chinese. It's a bit like, e.g. Mexico claiming Sicily because it was once ruled by Spain, or Algeria claiming Romania because it was once ruled by the Ottomans. On that basis, China could claim everything Genghis Khan conquered, or India could claim the eastern USA.