Charles XII of Sweden moved south into Ukraine to avoid bitter winter conditions and to join forces with the rebel Cossack leader Mazepa. After besieging the fortress at Poltava he was met by a Russian army of 70,000, and outnumbered two to one. Tsar Peter organized a series of defensive positions to protect his camp against the Swedish attack, which was launched under cover of darkness early on 28 June. The surprise Swedish attack was detected and hampered by a disorganized command as Charles had sustained a serious injury on 20 June. Most of the Swedish forces bypassed the Russian defences, ready to attack the main camp, but General Roos’s unit tried to overrun the defences directly and retreated after sustaining heavy losses. Peter then sent his main force west where they managed to outflank the depleted Swedes, and score a decisive Russian victory after the remaining Swedish forces fled.