During the German invasion of the Grand Duchy of Luxembourg, that began 10 May 1940, all the Grand Ducal Family and members of the Luxembourg Government left the national territory to evade capture. Via Belgium, France and Portugal, the family took refuge in the United States and then in Canada, while the Grand Duchess Charlotte and establish Her government in London.
In Autumn 1940, from London, thanks to the cooperation of the BBC, the Grand Duchess Charlotte regularly sent radio airwave messages of support to the people of Luxembourg. In November 1942, Prince Jean, Hereditary Grand Duke of Luxembourg, joined as a volunteer in the Irish Guards Regiment and received officer training at the Royal Military College, Sandhurst. On 28 July 1943 he was promoted to Lieutenant.
The Hereditary Grand Duke landed June 11, 1944 near Bayeux. He took part in Operation Overlord in Normandy and was attached to the Staff of the 32nd Brigade of the Guards Armoured Divison. Under the Irish Guards uniform, the Hereditary Grand Duke Jean participated in the battle between Caen and in Brussels on 3 September 1944. he was later detached from his unit to join his father, Prince Felix.
On 10 September 1944, Prince Felix and his son Jean entered Luxembourg. On 13 September 1944. Jean rejoined his unit and participated in subsequent operations including the landing operation "Market Garden" around Arnhem and fightingin the offensive "von Rundstedt." At the end of January 1945 he participated in the capture of Reichswald northwest of Wesel.
Hereditary Grand Duke Jean continued the campaign with allied forces in Germany until the end of hostilities. With members of the Government, the Grand Duchess Charlotte, thanks to special relations with U.S. President FD Roosevelt, worked to ensure the recognition of the independence of the country. April 14, 1945, the Grand Ducal Family and members of the Government finally returned to the free Grand Duchy of Luxembourg. They were greeted by an entire country cheering, victorious. Grand Duchess Charlotte immediately spent a tireless month visiting devastated areas, while also tirelessly addressing the concerns of war victims.