Finally, Prussia had remained at peace with France the previous year, though it did come close to joining the Allies in the Third Coalition. A French corps led by Marshal Bernadotte had illegally violated the neutrality of Ansbach in Prussian territory on their march to face the Austrians and Russians. Anger by Prussia at this trespass was quickly tempered by the results of Austerlitz, and a convention of continued peace with France was signed two weeks after that battle at Schönbrunn. This convention was modified in a formal treaty two months later, with one clause in effect promising to give Hanover to Prussia in exchange for Ansbach's being awarded to France's ally Bavaria. In addition, on 15 March 1806 Napoleon elevated his brother-in-law Marshal Joachim Murat to become ruler of the Grand Duchy of Berg and Cleves (acquired from Bavaria in return for its receiving Ansbach). Murat exacerbated Prussian enmity by tactlessly ejecting a Prussian garrison that was stationed in his newly acquired realm, prompting a stern rebuke from Napoleon. Relations between France and Prussia quickly soured when Prussia eventually discovered that Napoleon had secretly promised to return sovereignty of Hanover back to Britain during his abortive peace negotiations with the British. This duplicity by the French would be one of the main causes for Prussia declaring war that autumn.