Panzer IVs comprised around half of the available German tank strength on the Western Front prior to the Allied invasion of Normandy on 6 June 1944. Most of the 11 panzer divisions that saw action in Normandy initially contained an armored regiment of one battalion of Panzer IVs and another of Panthers, for a total of around 160 tanks, although Waffen-SS panzer divisions were generally larger and better equipped than their "Heer" counterparts. Regular upgrades to the Panzer IV had helped to maintain its reputation as a formidable opponent. The bocage countryside in Normandy favored defense, and German tanks and anti-tank guns inflicted very heavy casualties on Allied armor during the Normandy campaign, despite the overwhelming Allied air superiority. German counter-attacks were blunted in the face of Allied artillery, infantry-held anti-tank weapons, tank destroyers and anti-tank guns, as well as the ubiquitous fighter-bomber aircraft. The side skirt armor could predetonate shaped charge anti-tank weapons such as the British PIAT, but could be pulled away by rugged terrain. German tankers in all theaters were "frustrated by the way these skirts were easily torn off when going through dense brush".