On hearing Napoleon had returned to power in 1815, d'Auvergne went to Brussels and marched to war with a small regiment formed in the colours of Bouillon. Nine days before the Battle of Waterloo, d'Auvergne discovered that the Congress of Vienna, which was allowed by the Treaty of Paris to rewrite the map of Europe, had decided to form the United Kingdom of the Netherlands as a buffer state along the northern border of France. This meant there was no place for an independent principality and the Duchy of Bouillon would be annexed to the Grand Duchy of Luxembourg (at that time considered part of the Kingdom of the Netherlands due to the personal union of the two states).