In 1799, he was appointed to Jean-Baptiste Jourdan's general staff of the Army of the Danube, garrisoned initially in Strasbourg. After crossing the Rhine in early March, the Army engaged Archduke Charles' Austrian troops at the battles of Ostrach and Stockach. Following defeat at Ostrach, the army was reorganized with the Army of Helvetia, under command of André Masséna. Hastrel received a staff appointment in Milan.