Nyorai kupfuurira kweiyi ndima - Muna May 1940, Belgium yakapambwa neNazi Germany. Zvichitevera kukundwa kweBelgium uye kutanga kwekugara kweGerman, mukuru wePOB<unk>BWP Henri de Man akazivisa kuputswa kwebato rezvematongerwo enyika sechikamu chemutemo wekubatana. Van Acker akasiya upenyu hwevoruzhinji. Muna 1941, zvisinei, akabatanazve nebato risiri pamutemo rezvematongerwe enyika uye sangano rezvematongerwe enyika risingazivikanwi pasi pe "nom de guerre" "Monsieur André" uye akafamba-famba munyika yose achiita kuonana nezvikamu zvebato. Pakusunungurwa kweBelgium
munaGunyana 1944, Van Acker akabuda semutungamiri webato rakatevera POB<unk>BWP, Belgian Socialist Party ("Belgische Socialistische Partij", PSB-BSP).