In 1803 he was made governor and garrison commander at Graudenz, a fortress and city along the Vistula in West Prussia. During the War of the Fourth Coalition much of Prussia was occupied after the Battle of Jena–Auerstedt and the Fall of Berlin in 1806. King Frederick William III of Prussia himself relocated to East Prussia, staying at Gradenz for a day during his journey. While in the follow-up months many isolated Prussian fortresses and detachments surrendered or dissolved, Courbière and his roughly 5,700-strong garrison stood firm on 22 January 1807, when French troops took the city of Graudenz and began blockading the fortress. The French forces consisted of French and Hessian troops as well as contingents of Polish insurgents. Initially commanded by General Marie François Rouyer, their numerous demands to surrender were refused by Courbière. Then, after briefly leaving the field, they reoccupied the city and, by now commanded by General Johann Georg von Schäffer-Bernstein, started besieging the fortress again. In March Napoleon, impending to besiege Danzig (situated down on the end of the Vistula), sent General Anne Jean Marie René Savary to demand surrender again. In one of his depeches Savary argued that there was no King of Prussia anymore as he had left his lands in French hands. However, in reference to King Frederick William Courbière refused to surrender and verbally answered "While there may be no King of Prussia anymore there still is a King of Graudenz.", though the exact wording or meaning is somewhat disputed. In the following months the siege was intensified, the French got more reinforcements and after Danzig fell command was briefly given to Claude Victor-Perrin, Duc de Belluno before Rouyer took over again.