In the first half of the 18th century, the Russian Empire constructed the Irtysh line, a series of forty-six forts and ninety-six redoubts, including Omsk (1716), Semipalatinsk (1718), Pavlodar (1720), Orenburg (1743) and Petropavlovsk (1752), to prevent Kazakh and Oirat raids into Russian territory. In the late 18th century the Kazakhs took advantage of Pugachev's Rebellion, which was centred on the Volga area, to raid Russian and Volga German settlements. In the 19th century, the Russian Empire began to expand its influence into Central Asia. The "Great Game" period is generally regarded as running from approximately 1813 to the Anglo-Russian Convention of 1907. The tsars effectively ruled over most of the territory belonging to what is now the Republic of Kazakhstan.