During the German invasion of Poland, which started World War II, the town was bombed by the Germans on September 5, 1939, and then captured by German troops on September 10. Afterwards it was annexed into the newly formed province of Reichsgau Wartheland. Under German occupation, the town was subjected to intense Germanisation policies, Polish organizations were dissolved, churches were closed. The town was renamed to "Grenzhausen" in attempt to erase traces of Polish origin. Between 1939 and 1941 most Polish inhabitants were expelled from Słupca for Germans to take their place as part of the "Heim ins Reich" Nazi policy. Several Poles from Słupca, including policemen and a pre-war mayor of the town, were murdered by the Russians in Katyn, Tver and Kharkiv during the large Katyn massacre in April–May 1940. From 1941, around 1,800 Jews of Słupca - men, women, and children, were murdered by the Nazi Germans. Słupca remained under German occupation until January 21, 1945.