Brunswick's army, composed mostly of Prussian veterans, crossed into French territory on 19 August and easily took the fortresses of Longwy and Verdun. But at the Battle of Valmy on 20 September 1792 they came to a stalemate against Dumouriez and Kellermann in which the highly professional French artillery distinguished itself. Although the battle was a tactical draw, it bought time for the revolutionaries and gave a great boost to French morale. Furthermore, the Prussians, facing a campaign longer and more costly than predicted, decided against the cost and risk of continued fighting, and determined to retreat from France to preserve their army.