On 20th December, the US 101st Airborne was sent to Bastogne to support the 10th Armoured and other units under Gen. McAuliffe holding the vital road junction and town. They were soon completely surrounded by Manteuffel's Fifth Panzer Army but refused to give it up (McAuliffe famously replied 'Nuts!' to the German commander's offer of terms).
Running short of all forms of supplies and with many wounded, they held out until Patton's 3rd Army manages to punch a hole through to them, effectively preventing the 5th Panzer from making the essential progress needed to keep the offensive rolling.