In an effort to bolster command and control of the northern shoulder, Eisenhower gave Field Marshal Bernard Law Montgomery, commander of the 21st Army Group, command of all troops north of the German advance on 20 December. This was done in part because Montgomery controlled an uncommitted reserve, the British XXX Corps. This made little difference to the American troops defending Elsenborn Ridge, however. On the same day, Sixth Panzer Army made several all-out attempts to smash the American lines. It committed artillery, tanks, infantry, and self-propelled guns, supported by a Jagdpanther battalion and remnants of the Panzer IV and Jagdpanzer IV units. They unsuccessfully attacked at 09:00, 11:00 and 17:30. They were met by a deluge of American artillery and anti-tank gun fire from units of the American 1st Infantry Division, backed up by strong artillery support. All attacks were repelled with heavy losses.