In the 1840s and 1850s, the Russians extended their control into the steppes, where after capturing the Khokandi fortress of Aq Masjid in 1853, they sought to fortify a new frontier along the Syr Darya River, east of the Aral Sea. The new fortresses of Raim, Kazalinsk, Karmakchi and Perovsk were islands of Russian sovereignty in a desolate landscape of salt marshes, swamps and deserts subject to extreme cold and heat. Supplying the garrison proved difficult and expensive, and the Russians became dependent on the Bukhara grain merchants and Kazakh cattle breeders and fled to the outpost in Kokand. The Syr Darya border was a fairly effective base for eavesdropping on Russian intelligence, repelling attacks from Khokand, but neither Cossacks nor peasants were convinced to settle there, and the costs of occupation far exceeded the income. By the end of the 1850s, there were calls for a withdrawal to the Orenburg front, but the usual argument - the argument of prestige - won out, and instead the best way out of this "particularly painful place" was an attack on Tashkent.