Josias joined the Habsburg military as Colonel in 1759. After participating in the Seven Years' War, he 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 success in the Battle of Focşani (August 1, 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. Soon after that, he became a Field General.