Complex sentence: Heraclius became concerned when he heard of this: his army was severely depleted by the desertion of the Turkic contingent and was concerned that Rhahzadh's reinforcements could tip the scale.
Split into simpler sentences: Heraclius became concerned when he heard of this. His army was severely depleted by the desertion of the Khazar contingent and was concerned that Rhahzadh's reinforcements could tip the scale.