both to great effect. Commonwealth's cavalry, including the Winged Hussars, significantly restricted Russian mobility, forcing them to stay in their trenches. In a series of fierce engagements, Commonwealth forces gradually overran the Russian field fortifications, and the siege reached its final stages by late September. On 28 September 1633, Commonwealth forces took the main Russian supply points, and by 4 October the siege had broken. Shein's army retreated to its main camp, which was in turn surrounded by Commonwealth forces in mid-October. The besieged Russians waited for relief, but none arrived, as Commonwealth and Cossack cavalry had been sent to disrupt the Russian rear. Some historians also cite dissent and internal divisions in the Russian camp as responsible for their inaction and ineffectiveness. (Jasienica blames the Russian warlords, and Parker the foreign mercenaries.) The Tatar invasion threatening the south Russian borderlands was a contributory factor, with many soldiers and boyars from those regions deserting the Russian camp to return to protect their homeland. Some foreign mercenaries also deserted to the Commonwealth side. Shein began surrender negotiations in January 1634, and by February they were in full swing. The Russians finally signed a surrender treaty on 25 February 1634, and on 1 March they vacated their camp. (Some scholars, such as Rickard and Black, give the date of 1 March for Shein's capitulation.) Under the surrender terms, the Russians had to leave behind most of their artillery but were allowed to retain their banners after a ceremony in which they were laid before King Władysław. They also had to promise not to engage Commonwealth forces for the next three months. Shein's forces numbered around 12,000 at the time of their capitulation, but over 4,000, including most of the foreign contingent, immediately decided to defect to the Commonwealth. Other engagements Several other towns and fortresses in the region were the site of smaller battles. Russian forces captured several significant locations during their advance in 1632, but Nagielski speculates that the delay in the arrival of their main force and artillery at Smolensk caused by this dilution of effort may have cost them the siege and consequently the war. In July 1633, the Russians took the towns of Polatsk, Velizh, Usvyat, and Ozerishche. Polatsk was the scene of particularly heavy fighting as the Russians captured the city and part of the fortress. However, attacks on Vitebsk and Mstsislaw were successfully repulsed. Polish forces