The siege of Longwy (20-23 August 1792) was the first military success during the Austrian and Prussian invasion of France at the start of the War of the First Coalition. The Allied army, under the overall command of the Prussian field marshal Karl Wilhelm Ferdinand, duke of Brunswick, was to advance towards Paris from the direction of Luxembourg, a route that required the capture of the border fortresses of Longwy and Verdun.
The attack on Longwy was commanded by General Clairfait, a capable Walloon solider in the Imperial Austrian Army. The fortress was invested on 20 August. The two day long bombardment started on the following day, and on 23 August the garrison surrendered. The fortress is generally accepted to have been in a poor condition before the short siege, but the low morale of the 3,500 strong French garrison almost certainly played a bigger part in the rapid collapse in resistance.
After capturing Longwy, the Allies moved on to attack Verdun, which fell on 2 September, after an equally short siege. After these successes, the Allied army advanced slowly towards Paris, but after suffering a defeat at Valmy on 20 September Brunswick was forced to retire back across the French border. Longwy was abandoned to the French on 19 October.