The Battle of Modlin took place during the 1939 German invasion of Poland at the beginning of the Second World War. Modlin Fortress was initially the headquarters of the Modlin Army until its retreat eastwards. From 13 to 29 September 1939, it served as a defensive citadel for Polish forces under the command of General Wiktor Thommée against assaulting German units. The fighting was closely linked with the strategic situation of the Battle of Warsaw.