Upon the outbreak of World War II Prince Xavier resumed his duties in the Belgian Army, serving as major in his old artillery unit. As the Germans advanced swiftly, the Belgians were pushed back to Flanders, towards the English Channel. Incorporated into the French troops, the regiment was withdrawn into Dunkerque. In the mayhem that followed, the Belgians did not make it to the British evacuation ships and Don Javier became a German POW.