Jamukha assembled 30,000 men and moved in an arc from the north to flank Temüjin's position. The two forces were evenly matched but Temüjin's side suffered slightly worse than Jamukha, and was forced to retreat to a defensible pass called Jerene near the Onon River. Despite Jamukha's victory, his harsh treatment of captives disgusted his allies so much that they defected to Temüjin, bringing with them 10,000 men. With less than 20,000 men at his side, Jamukha was no longer able to challenge Temüjin on the upper Kherlen River, and retreated further east