Complex sentence: Jalal al-Din counterattacks, nearly breaching the center of the Mongol army, while Genghis sends a contingent of ten thousand men around the mountain and flanks Jalal al-Din's army.
Split into simpler sentences: Jalal al-Din counterattacked, and nearly breached the center of the Mongol army. Genghis then sent a contingent of ten thousand men around the mountain to flank Jalal al-Din's army.