Cardinal Richelieu called him la Guerre ("War") and commandeered his services which had proved valuable to Gustavus Adolphus, one of the renovators of new cavalry tactics in the West. Fortuitously, he was present as a cavalry commander under the young duc d'Enghien, Louis II de Bourbon, the Grand Condé several days after the death of Louis XIII; Gassion was a crucial factor in the French success at Rocroi against combined Habsburg forces. De Gassion's role was gracefully acknowledged by Condé.