The Germans achieved a total surprise attack on the morning of 16 December 1944, due to a combination of Allied overconfidence, preoccupation with Allied offensive plans, and poor aerial reconnaissance due to bad weather. American forces bore the brunt of the attack. The Germans had attacked a weakly defended section of the Allied line, taking advantage of heavily overcast weather conditions that grounded the Allies' superior air forces. Fierce American resistance on the northern shoulder of the offensive, around Elsenborn Ridge, and in the south, around Bastogne, blocked German access to key roads to the northwest and west that they counted on for success. Columns of armor and infantry that were supposed to advance along parallel routes found themselves on the same roads. This congestion, and terrain that favored the defenders, threw the German advance behind schedule and allowed the Allies to reinforce the thinly placed troops. The farthest west the offensive reached was the village of Foy-Nôtre-Dame, south east of Dinant, being stopped by the U.S. 2nd Armored Division on 24 December 1944.[15] Improved weather conditions from around 24 December permitted air attacks on German forces and supply lines, which sealed the failure of the offensive. On 26 December the lead element of Patton's U.S. Third Army reached Bastogne from the south, ending the siege.

https://en.wikipedia.org/wiki/Battle_of_the_Bulge