The Battle of Arnhem was part of Operation Market Garden, an attempt to secure a string of bridges through the Netherlands. At Arnhem the British 1st Airborne Division and Polish 1st Independent Parachute Brigade were tasked with securing bridges across the Lower Rhine, the final objectives of the operation. However, the airborne forces that dropped on 17 September were not aware that the 9th SS and 10th SS Panzer divisions were also near Arnhem for rest and refit. Their presence added a substantial number of Panzergrenadiers, tanks and self-propelled guns to the German defences and the Allies suffered heavily in the ensuing battle. Only a small force managed to hold one end of the Arnhem road bridge before being overrun on 21 September. The rest of the division became trapped in a small pocket west of the bridge and had to be evacuated on 25 September. The Allies failed to cross the Rhine, which remained under German control until Allied offensives in March 1945.