In 1743, when the 16 year-old Charles de Castries received a commission as Lieutenant in the Régiment du Roi, Infanterie, it would have been hard to predict just how long a career he would have in the public service, or to imagine the series of brilliant successes, promotions, and royal decorations he would garner or the intense devotion he would hold for the crown even under the direst circumstances. During the War of Austrian Succession, Castries distinguished himself as an officer in the Régiment du Roi, Cavalrie, and for his part at the Battle of Dettingen and the Siege of Maastricht, he was rewarded with promotion to Maréchal de Camp and assignment as commander of Corsica (1756).
It was during the Seven Years' War, however, that Castries truly established an enduring military reputation. Attached to the Armée de l'Allemagne under the Prince de Soubise, Castries received three blows of a saber to his head at the Battle of Rosbach, but nevertheless, he refused to retire from action until the end of the battle. Returning to service in 1758 as commander of a separate corps in the same army, he fought at Lutzelberg, Saint-Goar, at the capitulation of the castle of Rhinfelds, and at Minden. His military career reached its zenith with a stunning defeat delivered to the forces of the hereditary Prince of Brunswick at Clostercamp in 1760. After five hours of intense fighting and sustaining tremendous casualties, the Duke's army was forced to retreat across the Rhine and to abandon the siege of Wesel, a key to controlling the countryside between the Meuse and Rhine. A monument was erected on the battlefield at Clostercamp in Castries' honor, and Louis XVI named him Chevalier des Ordres du Roi for his role in this pivotal engagement.
After participating in the German campaigns of 1761 and 1762, Castries was appointed, successively, as Commander in Chief of the Gendarmerie and Governor General in Flanders and Hainault. Skillfully playing the court and using his own talent and the influence of his sometime lover, Mme. de Blot, Castries rose to an appointment as Ministre de la Marine in 1780, and was honored with a promotion to Maréchal de France in 1783. Castries' ministry came at a critical period, following the massive naval buildup overseen by Choiseul and coinciding with the final stages of the American Revolution, the Peace negotiations between Britain and her former colonies, and the transfer of a number of Caribbean colonies between Britain and France. Castries is credited with orchestrating the highly successful naval campaign of 1781 and elevating Suffren de Saint Tropez to command of French naval forces, as well as instituting a variety of progressive reforms in the French Navy, including the de facto introduction of a new naval code. Castries resigned his ministry in 1787 in the midst of court intrigues and attempts to limit naval spending.
At the start of the French Revolution, Castries cast his lot with the most militant Royalists, and left France to take refuge with his former rival, the Duke of Brunswick. He became a key figure in assisting French Royalists to escape and subsist in exile, and he was important in coordinating the efforts of the various counterrevolutionaries. Castries joined the Conseil des princes in August, 1791, and the following year led a division of the Army of Princes in an invasion of the Champagne region that occupied the heights along the Moselle River near Thionville. Though forced to retreat and faced with limited and declining prospects of success, Castries remained firm in his opposition to the Republic and was rewarded in 1797, with his last royal appointment, helping the comte de Saint-Priest to direct the cabinet of Louis XVIII. Castries died at Wolfenbuttel on the 12th of January, 1801, and is buried at Brunswick, Germany, near the memorial erected by the Duke to the victor at Clostercamp.