As soon as the Russian conquest of the Caucasus was completed in the late 1850s, the Russian Ministry of War began to send military forces against the Central Asian khanates. Three major population centers of the khanates - Tashkent, Bukhara, and Samarkand — were captured in 1865, 1867, and 1868, respectively. In 1868 the Khanate of Bukhara signed a treaty with Russia making Bukhara a Russian protectorate. Khiva became a Russian protectorate in 1873, and the Khanat of Kokand finally was incorporated into the Russian Empire, also as a protectorate, in 1876.