Mongols evidently believed it was a considerable force, given the extreme lengths the Mongols went to dislocate its formations. The move by the Rus' army was detected by the Mongols, who were on the east side of the Dnieper River waiting for reinforcements from Jochi, Genghis Khan's eldest son, who was campaigning around the Aral Sea. Jochi, however, had become ill, which meant no reinforcements would be coming. At the same time, the Rus' attempted to trap the Mongols. The princes of Halych and Volhynia transported their armies south down the river, while the princes of Kiev and Chernigov advanced north up the river, and the army of Kursk advanced from the front. At the same time, the Cumans attempted to attack the Mongol army's rear. When Jebe learned of this, he sent 10 envoys to the Prince of Kiev. The envoys stated that the Mongols had no feud with the Rus and were only attacking the Cumans; they added that the Mongols were marching east, away from the Rus' cities. Mstislav of Kiev had the envoys executed, and the Mongols responded by sending another set of ambassadors, who declared war. When Jebe and Subutai heard of the Rus' movements, they began moving east, away from the Rus, which was the only direction in which they could move. However, they left a rearguard of 1,000 under the command of an officer, Hamabek, to report of the Rus movements and act as bait. Soon, Mstislav the Bold reached the river opposite the rearguard, and it became apparent that no prince had been appointed commander-in-chief. Thus, all the princes could act as they pleased. Eventually, Mstislav crossed the river under heavy arrow fire. When the Rus landed, however, their numbers were too great, and the Mongols were killed to the last man. Rus' attack After drawing out the Rus armies for nine days in a feigned retreat, the Mongol army turned to face their pursuers along the Kalka River (the river's location is currently unknown, but it is thought to be the Kalchyk which flows into the Sea of Azov). The Russian primary sources give only a very general account of the battle itself and the pursuit of the princes back across the steppe. The chronicles name which princes took part and which died, but not much more in terms of the size of armies or casualties. As to the actual battle