Battle of Frankfurt an der Oder The Battle of Frankfurt an der Oder on 13 / 15 April 1631 was a battle of the Thirty Years ' War , fought between the Swedish Empire and the Holy Roman Empire for the strategically important , fortified Oder crossing Frankfurt an der Oder , Brandenburg , Germany . The town was the first major Imperial stronghold attacked by Sweden outside the Duchy of Pomerania , where Sweden had established a bridgehead in 1630 . After a two - day siege , Swedish forces , supported by Scottish auxiliaries , stormed the town . The result was a Swedish victory . With the subsequent clearance of nearby Landsberg ( Warthe ) ( now Gorzow ) , Frankfurt served to protect the Swedish army ' s rear when Gustavus Adolphus of Sweden proceeded further into Central Germany . Prelude . Swedish king Gustavus Adolphus started to intervene in the Thirty Years ' War by supporting Stralsund against Wallenstein in 1628 , and landed in Pomerania in June 1630 . With the central parts of the Duchy of Pomerania , Sweden had gained a bridgehead in the Holy Roman Empire ' s northeasternmost corner , while the rest of the empire was occupied by forces of the Catholic League and Ferdinand II , Holy Roman Emperor . Except for Magdeburg , which had allied with Sweden on 1 August , the German Protestant states distrusted Gustavus Adolphus and hesitated to enter an alliance . In January 1631 , Swedish forces garrisoned in the Pomeranian bridgehead proceeded southwards , and sacked the Pomeranian towns of Gartz ( Oder ) and Greifenhagen ( now Gryfino ) near Pomerania ' s border with Brandenburg . Further southward advances along the Oder into the territory of George William , Elector of Brandenburg followed , and on 23 January 1631 , Sweden allied with France in the Treaty of Bärwalde , concluded in Brandenburgian Bärwalde ( now Mieszkowice ) . Siege and storm of Frankfurt . The Swedish forces , commanded by Gustavus Adolphus , were supported by Scottish auxiliaries commanded by John Hepburn and Robert Monro . They laid siege on the town for two days , and stormed it on the second day . The assault was successful and resulted in the sack of the town . The success was in part due to internal quarrels in the defending force — mercenaries who had not been paid refused to fight without receiving their pay first . The defenders were " slaughtered [ . . . ] where they stood " and suffered 3 , 000 deaths , compared to 800 casualties on the Swedish side . Many deaths occurred when the town was looted . Aftermath . Scottish major general in Swedish service John Leslie was appointed governor of the town and gave orders to have its defenses strengthened and the thousands of bodies buried . The latter task was achieved by digging mass graves for over a hundred bodies each ; after six days , all dead had been buried . John Leslie was soon succeeded as Frankfurt ' s governor by another Scot , James MacDougal , who was in turn succeeded by a third Scot , Alexander Leslie . Frankfurt served to protect the rear of the advancing Swedish army . The other major town in northeastern Brandenburg , Landsberg ( Warthe ) ( now Gorzow ) was taken on 23 April . Subsequently , George William , Elector of Brandenburg was forced into treaties with Sweden on 14 May , 20 June , and 10 September 1631 , which put Sweden in charge of the Brandenburgian military capacities , but did not have the status of an actual alliance . Throughout 1631 , Gustavus Adolphus of Sweden advanced into central Germany , and while Magdeburg was lost in May and Gustavus Adolphus was hard - pressed at Werben in July , the subsequent victory at Breitenfeld in September paved the way for his advance into southern Germany .