 Altogether, before being finally liberated by the arrival of Allied forces, he had passed through no less than seven Nazi prisons and camps.Directly upon being freed from Nazi imprisonment, Princen joined the "Stoottroepen Regiment Brabant" (Brabant Stormtroop Regiment), based in the southern Dutch province of Brabant.In 1945, he also worked for the newly founded "Bureau voor Nationale Veiligheid" (Bureau of National Security ), forerunner of the present Dutch Security Service – at the time mainly involved in hunting down collaborators and war criminals, but also keeping under surveillance those natives of the Dutch Indies, resident in the Netherlands, who were sympathetic with the rebellion against Dutch rule that was spreading in their homeland.