The town eventually returned to Poland when the country regained its independence in 1918. During World War II, Ślesin was occupied by the Wehrmacht on 13 September 1939, annexed to Nazi Germany from 1939 to 1945 and administered as part of the newly formed province of Reichsgau Wartheland. The town was liberated by the Red Army on 20 January 1945 during the Vistula–Oder Offensive, and restored to Poland.
The town's history is marked by its return to Poland in 1918 after gaining independence. During World War II, Ślesin was occupied by German forces from 1939 to 1945 and administered as part of the Reichsgau Wartheland province. The town was liberated by Soviet troops on January 20, 1945 during the Vistula-Oder Offensive, and subsequently returned to Polish control.