Frederick played an important role as a military leader of the Dutch Republic during the era of the Patriots as a negotiator with the Austrian emperor Joseph II, to dismantle the Barrier treaties 1709-1715. As commander of the Legion of the Rhine, formed by Count of Salm during the Kettle War in 1784, his troops were responsible for the defense of the republic. The troops included eight companies of Cavalry, two companies of Jaegers, and six companies of Foot, including sharpshooters. In 1786, Holland employed him to maintain an army, but six months later dismissed his regiment to save money. In Amsterdam a fund was set up to support his troops so that Von Salm, eager to replace the Stadtholder, could remain. On May 12, 1787 Frederick led the army to Utrecht, to protect the Patriot occupation of the "democratic Eldorado". On the 28 June, he marched to Woerden to capture princess Wilhelmina of Prussia, the wife of the Prince of Orange. Early September 1787, the Gelder Jaeger corps occupied Makkum on the initiative of Court Lambertus van Beyma. When Frederick William II of Prussia invaded the Dutch Republic, Frederick left Utrecht three days later, without giving battle. His regiment retreated through Amsterdam to Weesp, and Frederick endured heavy criticism. There are different versions of what happened next. He may have remained hidden for a few months in the house ofHenry Hope. Possibly, he secretly left the city of Amsterdam within a week and stopped in Jever, but as East-Frisia was Prussian, that is not very likely. He may have fled to his brother at Grumbach in the Rhineland-PalatinateIn the summer of 1788, one of the leading patriots Pieter Paulus refused to admit him on a visit to Paris. In 1791, Quint Ondaatje, his personal assistant, wrote him an apology. In 1794, he was guillotined together with Alexandre de Beauharnais, the lover of Amalie Zephyrine van Salm-Kyrburg, for their ties with the "Ancien Régime".
Frederick commissioned the construction of the Hôtel de Salm in Paris, where Madame de Stael gave her soirees in 1797. From 1804, the Légion d'honneur resided in the building, but it was destroyed by fire in 1871. The structure was rebuilt and is now known as the Palais de la Légion d'Honneur. Henry Hope wanted to copy the Hôtel de Salm in Haarlem, for his Villa Welgelegen.
