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.