 After the May 10, 1940 invasion and occupation of the Netherlands by Nazi German forces, Van Dam initially continued to work as before but, as anti-Jewish measures increased throughout 1941, he went into hiding in Blaricum, where he continued to work.He attempted to escape and managed to reach Vichy France but returned to the Netherlands several months later in an effort to attempt an escape with Alice Weil and her daughter, Jacqueline de Jong, to Weil's native Switzerland.In the autumn of 1942 they managed to reach the French Alps but were arrested as a result of betrayal.