When the Siege of Mantua was briefly raised in early August 1796, Lauer supervised repairs to its fortifications. On 19 August, Emperor Francis II ordered Wurmser to make another attempt to relieve the town and named Lauer as his chief-of-staff. When drawing up the plan of attack, Lauer assumed that losses would prevent the French from quickly reacting to an Austrian offensive. This belief proved mistaken when Bonaparte hurled three divisions north into the upper Adige River valley. After overwhelming Paul Davidovich's covering force at the Battle of Rovereto, the French commander sent his troops marching east, then south down the Brenta River valley. Bonaparte crushed Wurmser at the Battle of Bassano on 8 September, then followed him as he marched toward Mantua. Ultimately, Wurmser became trapped in Mantua along with nearly 30,000 soldiers. "To his credit, Lauer thereafter proved the key man during the very stubborn defense of Mantua." The fortress finally surrendered on 2 February 1797.