In the War of the Austrian Succession, he took part in the storming of Prague in 1742, and was made a brigadier. In 1744 and 1745 he saw further service on the Rhine, and he succeeded his father as 2nd duc de Broglie on the old duke's death in 1745. He was made a "Maréchal de Camp," and he subsequently served with Marshal de Saxe in the Low Countries, and was present at Roucoux, Val and Maastricht. At the end of the war, he was made a lieutenant-general.