Josias joined the Habsburg military as Colonel in 1759, participated in the Seven Years' War, and rose to the rank of Lieutenant Field Marshal by 1773. In the Russo-Turkish-Austrian war of 1788, he commanded an army corps under Freiherr von Laudon, occupying Moldavia, capturing Khotyn in Bessarabia and sharing in Aleksandr Suvorov's victory in the Battle of Focșani (1 August 1789). Having completely beaten the main Ottoman army under Grand Vizier Koca Yusuf Pasha in the Battle of Rymnik, he captured the greater part of Wallachia, including Bucharest, being welcomed by the population after the flight of Prince Nicholas Mavrogenes ("see History of Bucharest"), and soon after becoming a field marshal.