Spearheading the attack were two panzer armies composed largely of Waffen-SS divisions.[241] The battle groups found advancing through the forests and wooded hills of the Ardennes difficult in the winter weather, but they initially made good progress in the northern sector. They soon encountered strong resistance from the US 2nd and 99th Infantry Divisions. By 23 December, the weather improved enough for Allied air forces to attack the German forces and their supply columns, causing fuel shortages.