In May 1940, Belgium was invaded by Nazi Germany. Following the Belgian surrender and the start of the German occupation, the head of the POB–BWP Henri de Man announced the dissolution of the socialist party as part of a policy of collaborationism. Van Acker retreated from public life. In 1941, however, he rejoined the illegal socialist party and underground trade union movement under the "nom de guerre" "Monsieur André" and travelled around the country making contact with party sections. At the Liberation of Belgium in September 1944, Van Acker emerged as the head of the POB–BWP's successor party, the Belgian Socialist Party ("Belgische Socialistische Partij", PSB-BSP).