In 1792, he was initially placed in command of the 50,000 Austrian forces in the Upper Rhine Valley. In August, his forces crossed the Rhine by Mannheim, and participated in the bombardment of Thionville, on the Moselle, in early September. Although the invading forces of the allies readily captured Longwy on 23 August and slowly marched on to Verdun, which was even less defensible than Longwy. The Duke of Brunswick now began his march on Paris and approached the defiles of the Argonne. In combination with the Army of Condé and Hessian troops, a portion of his force, 15,000, covered the left (southern) flank of the Prussian advance on Valmy.