שמואל ב, פרק י׳, פסוק י״ג

The momentum of a battle often hinges on a single tactical choice. In this military campaign, the opening strike determines the psychological fate of two enemy armies. Joab, acting as a skilled commander leading highly trained warriors, takes the initiative by launching a direct offensive against the Aramean army. The Arameans, however, are merely mercenaries. Lacking true commitment, they are unwilling to risk their lives in combat and immediately flee from the charging forces [ביאור שטיינזלץ].

The primary approach among commentators is that Joab's decision to attack the Arameans first shifts the entire course of the war. The Ammonites have placed their complete trust and reliance on the bravery of their Aramean allies. Witnessing the sudden defeat and retreat of the mercenary forces completely shatters their own confidence. Stripped of their morale, the Ammonites abandon the battlefield as well, retreating into the city [מלבי״ם, אברבנאל].