He joined the army in 1934 (infantry regiment 8 in Frankfurt/Oder) and participated in the Invasion of Poland in 1939. One year later he was dismissed from the army by Hitler's Prinzenerlaß, following the death of his elder brother Prince Wilhelm of Prussia, who was wounded in France in May 1940 and later died in a field hospital in Belgium.
Marriage and issue
On 29 December 1941, he married Baroness Maria Anna Sybilla Margaretha von Humboldt-Dachröden (9 July 1916 – 24 September 2003) in Oels, Schlesien. They divorced a little over a year later in early 1943 (the same year she would later give birth to Hubertus' first cousin Ernest Augustus of Hanover's illegitimate son), and on 5 June of that year, he married again to Princess Magdalena Reuss of Köstritz (20 August 1920 – 10 October 2009). They had two daughters:
