The Duchess of Galliera was disenchanted and quit Paris, leaving her mansion to the Austro-Hungarian Emperor, who made it his embassy in France. But the First World War found the two countries on opposite sides and, confiscated in 1919, the Hôtel Matignon was declared "enemy property". On 21 November 1922, after prolonged negotiations, France once more assumed ownership. During World War I, the hôtel was also the place where the philatelic collection of Philipp von Ferrary (the most valuable stamp collection ever assembled) was deposited when its owner, the son of the Duke of Galliera and an Austrian citizen, had to flee France in 1917. The collection was later broken up and sold by the French government after the war, as war reparations.