Near-complete surprise was achieved by a combination of Allied overconfidence, preoccupation with Allied offensive plans, and poor aerial reconnaissance. The Germans attacked a weakly defended section of the Allied line, taking advantage of a heavy overcast, which grounded the Allies' overwhelmingly superior air forces. Fierce 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 west that they counted on for success. This and terrain that favored the defenders threw the German timetable behind schedule and allowed the Allies to reinforce the thinly placed troops. Improved weather conditions permitted air attacks on German forces and supply lines, which sealed the failure of the offensive. In the wake of the defeat, many experienced German units were left severely depleted of men and equipment, as survivors retreated to the defenses of the Siegfried Line.