During the first Austrian attempt to relieve the Siege of Mantua, Guieu led a brigade in Pierre Sauret's division. Surprised by the attack of Peter Ott on 29 July, Guieu rallied 400 men and withdrew to the Palazzo Martinengo in the village of Salò. Though without food, Guieu withstood a siege by Joseph Ocskay's brigade until the 31st, when Sauret drove the Austrians away after a sharp action. During the Battle of Lonato on 3 August, Guieu led Sauret's division when the latter was injured. He recaptured Salò and attacked Gavardo from the east. Though driven back on Salò, he held his ground. The presence of a French division so close to the Austrian line of retreat helped Peter Quasdanovich make up his mind to withdraw in the early hours of 4 August.