 In 1852, a Russian military expedition under Muravyov explored the Amur, and by 1857 a chain of Russian Cossacks and peasants had been settled along the whole course of the river.In 1858, in the Treaty of Aigun, China recognized the Amur River downstream as far as the Ussuri River as the boundary between Russia and the Qing Empire, and granted Russia free access to the Pacific Ocean.The Sino-Russian border was later further delineated in the Treaty of Peking of 1860 when the Ussuri Territory (the Maritime Territory), which was previously a joint possession, became Russian.