In December 1944, and January 1945, the Battle of the Bulge put American ground forces on the operational defensive for the first time in Europe, as a German army group of 24 divisions (including ten panzer divisions with 1,500 armored vehicles) launched a major offensive in the Ardennes forest. The main thrust fell on the 99th and 2nd Infantry divisions which jammed the northern shoulder. A secondary attack hit two overstretched infantry divisions, both with attached towed tank destroyer battalions. Once contact was made, the towed guns were unable to reposition themselves or withdraw, and were often overrun by the enemy advance, or simply outflanked by infantry. The gun crews, unlike their self-propelled counterparts, had no protection against small-arms fire, and could easily be driven back by a squad of infantry. This lack of mobility was aggravated by the cold wet weather, and the rough conditions, which tended to bog down wheeled vehicles and immobilize fixed guns. Throughout the 1st Army as a whole, three-quarters of the tank destroyers lost were towed rather than self-propelled. One battalion, the 801st, lost 17 towed guns in just two days, while the M10-equipped 644th, which fought alongside it, found ideal opportunities for close-range ambushes and claimed 17 tanks in the same time period. It was clear that the towed guns had proved ineffective and, once the Ardennes offensive had been dealt with, the U.S. War Office approved Eisenhower's January 1945 request to convert all remaining towed units in the theatre to self-propelled guns.