 In September , Montgomery , now a field marshal , made his ambitious thrust across the Rhine and into Germany 's industrial heartland , codenamed Operation Market Garden , a priority for 21st Army Group . XXX Corps under Horrocks was to lead the ground assault , passing along a corridor held by airborne forces in order to link up with the British 1st Airborne Division in Arnhem within four days . In the event XXX Corps never arrived , and although 1st Airborne clung on to their tenuous position for a further five days , by 21 September almost three @-@ quarters of the division was destroyed or captured . Postwar analyses have been divided , some stressing a perceived lack of urgency on the part of Horrocks ' men , while others note that German defences in the area were severely underestimated by First Allied Airborne Army 's intelligence . Particularly important was the failure to identify the remnants of two SS Panzer divisions , which after Normandy had been sent to the Arnhem area for rest and refitting ; intelligence had stated that only " a few infantry units and between 50 and 100 tanks " were in the Netherlands . A series of counterattacks by Army Group B under Field Marshal Walter Model kept Horrocks ' units on the defensive , and delayed their advance by forcing the British to halt and secure their flank . The terrain over which Horrocks ' men had to move was unsuitable , restricting the vanguard ( Guards Armoured Division ) to a single narrow raised highway through flat or flooded countryside . Additionally , the Nijmegen Bridge , just 8 miles ( 13 km ) from Arnhem , was not captured by the 508th Parachute Infantry Regiment ( PIR ) on the first day as planned , and XXX Corps had to assist in its capture on their arrival in Nijmegen two days later , causing a further delay of 36 hours . Horrocks was not personally blamed for the operation 's failure ; during this period Major General James Gavin 's U.S. 82nd Airborne Division came under Horrocks ' command , and Gavin later wrote : 
