Immediately after leaving school the Prince was called up for military service, and served in Reserved Armoured Division no. 6 in Neuruppin until March 1945. He attended the Armoured Vehicle Training School at Viborg in Denmark for three months of this period.
From March to May 1945 he served with the 90th Panzer Division in Italy, but did not see active combat. He was captured by the Americans near Merano in early May and sent to a prisoner-of-war camp at Ghedi near Brescia, where he was employed as a driver and interpreter. In September 1945 the Americans transferred him to the US base Latimer Camp near Amersham in the United Kingdom to work as an interpreter. After his release in December 1945 Prince Claus returned to Hitzacker.
The Prince then had to sit the school-leaving examinations again as the certificate awarded him during the war was not officially recognised. He passed the examinations in Lüneburg in 1947.