He subsequently escaped from German-occupied Belgium and reached the United Kingdom, where he led a re-organisation of the Free Belgian military.

Following the German invasion of Belgium in August 1914 at the start of World War I, Piron, who had not finished his studies, was mobilised into the Belgian Army as a junior officer in the 2nd Regiment of the Line [fr].

[2] Rising through the ranks to major, he served with the Regiment of Frontier Cyclists [nl] and later at the headquarters of the 5th Army Corps at the outbreak of the Phoney War.

[2] Piron, however, refused to accept the Belgian surrender and succeeded in escaping from occupied Belgium via Vichy France and Spain to British Gibraltar.

Following his arrival in Britain, Piron was tasked with reforming the Belgian and Luxembourgish forces into an infantry battalion, an artillery battery, and an armoured squadron.

[2] The move followed a period of unrest among the Belgian troops which had culminated in a minor mutiny on 14 November, caused by inactivity and political infighting.

[5] According to Luc De Vos, a historian, "it was [...] indisputably Piron who played the major role in transforming the unit into a well-oiled military machine".

[1] After the Liberation, Piron became aide de camp to Charles, Count of Flanders, who became prince regent in 1944 and commander of the 2nd Infantry Regiment.

[2] While still serving as aide to the prince regent, Piron was promoted to command the Belgian Army of Occupation (Armée belge d'occupation, or ABO; Belgische Bezettingsleger, BBL) in Allied-occupied Germany in March 1946.