In the early years of the French Revolutionary Wars, Klenau distinguished himself at the Wissembourg lines, and led a battle-winning charge at Handschuhsheim in 1795. As commander of the Coalition's left flank in the Adige campaign in northern Italy in 1799, he was instrumental in isolating the French-held fortresses on the Po River by organizing and supporting a peasant uprising in the countryside. Afterward, Klenau became the youngest lieutenant field marshal in the history of the Habsburg military.