Russian expansion southwards was a process which extended over several centuries. In 1552, Tsar Ivan IV conquered the Muslim khanate of Kazan, which lay to the east of Muskovy, and two years later occupied Astrakhan, securing for the Russians the lower reaches of the Volga River and the northern shores of the Caspian Sea. The rapid introduction of Christianity and governmental institutions brought about the successful integration of those territories, but Russian preoccupations elsewhere, principally their penetration of Siberia, were such that it was not until the 1730s that they made any further moves southward. They then built a fortress at Orenburg and established a fortified ‘Orenburg Line’, running from the north-east of the Caspian Sea along the Ural River and eastwards to Omsk, where it met a West Siberian Line, which ran along the Irtysh River to Semipalatinsk and Ust-Kamenogorsk. The two lines extended over 2,000 miles in all and were guarded by some 20,000 men. The motives in building them were partly to help subdue the Bakshirs, whose territories lay east of the Volga and across the southern Urals, and partly to support the Kazakhs, who had sought Russian protection to the south. While the region remained restless, the strategy proved effective, supplemented by progressive colonisation and a gradual extension of control over the northern Kazakh steppe. For nearly a century, the Russians were faced with no significant threats and were under no pressure to move further: a natural frontier of a thousand miles of mostly empty steppe and desert separated them from the khanates of Central Asia, the nearest locations of settled populations of any significant size. There were just two aberrations: in 1717 a 3,500 man expedition sent to Khiva was massacred piecemeal on arrival; and in 1801 Tsar Paul, who was at that point becoming deranged, decided to send a force of 22,000 Cossacks, under the command of General Orlov, to march on India, despite there being little or no knowledge of the terrain or logistics involved. On the way, Khiva and Bokhara were to be reduced and India itself was to be made a Russian dependency. Even before the force had reached Orenburg, it had suffered severely from the bitterly cold winter conditions, and was only saved from complete annihilation when Paul was assassinated and urgent orders were sent for its return.