Treaty of Aigun
In May 1858 Nikolay Muravyov, Governor-General of Eastern Siberia, and the Qing general Yishan signed a treaty at Aigun making the land north of the Amur Russian and placing the land east of the Ussuri under joint rule.
Elisee Reclus, cartographer. W. Hughes, engraver. · Public domain · Source
Since the Treaty of Nerchinsk in the 17th century the Amur basin had been Qing territory. But in the mid-19th century Muravyov sent troops and settlers down the river and built posts all the way to its mouth.
In May 1858, as Anglo-French forces attacked the Dagu forts, Muravyov appeared at Aigun with gunboats and pressed the Heilongjiang general Yishan. The treaty signed on 28 May made some 600,000 km² north of the Amur Russian, and left the land east of the Ussuri under joint rule.
The Qing court tried to disown the treaty, but two years later the Convention of Beijing confirmed it and gave up the land east of the Ussuri as well.