By early September, Klenau's force had rejoined Wurmser's column and fought at the Battle of Bassano on 8 September. Here, the Austrians were outnumbered almost two to one by the French. As the Austrian army retreated, Bonaparte ordered a pursuit that caused the Austrians to abandon their artillery and baggage. Most of the third battalion of the 59th "Jordis", and the first battalion of the Border Infantry Banat were captured and these units ceased to exist after this battle. The Austrians lost 600 killed and wounded, and 2,000 captured, plus lost 30 guns, eight colors, and 200 limbers and ammunition waggons. Klenau was with Wurmser's column again as it fought its way to besieged Mantua and he participated in the combat at La Favorita near there on 15 September. This was the second attempt to relieve the fortress; as the Austrians withdrew from the battle, they retreated into Mantua itself, and from 15 September until 2 February 1797, Klenau was trapped in the fortress while the city was besieged. Following the Austrian loss at the Battle of Rivoli, north of Mantua, on 14–15 January 1797, when clearly there would be no Austrian relief for Mantua, Klenau negotiated conditions of surrender with French General Jean Sérurier, although additional evidence suggests that Bonaparte was present and dictated far more generous terms than Klenau expected. When the garrison capitulated in February, Klenau co-signed the document with Wurmser.