by section. Model's troops put up a strong resistance along the Dortmund–Ems Canal and the Sieg river-line, holding their ground from 4 April to 9 April and launching a counterattack against U.S. 75th and 95th divisions near Dortmund. For every German city or town that capitulated, another fought on for every building. Bürgermeisters of some German cities presented white flags to the invading U.S. troops, such as at Duisburg and Essen, while German troops at Dortmund, Wuppertal and Hamm fought fanatically for days to the complete exhaustion of all available potential. The presence of SS troops was a common element in most instances of all-out resistance. In the south, the attack of the U.S. III Corps and XVIII Airborne Corps on 5 and 6 April was delayed by German troops, who skilfully used the rugged terrain of the 80% forested Sauerland district to force the Americans to fight for every stream, wood and town. The Germans fought strongly for the city of Siegen to prevent the Americans from gaining access to open ground. The heavily outnumbered and outgunned Germans could ultimately do nothing more than delay the advancing enemy, who covered approximately 10 kilometers per day. By 11 April German combat strength had weakened to the extent that they were only defending roadblocks and built-up areas along main roads, supported by a few tanks and assault guns or 2 cm flak guns. At one point, the Germans covered a valley in a thick smokescreen, delaying the 7th Armored Division for some time. Throughout the battle, U.S. generals in the south failed to use their two armored divisions properly, attempting to unleash them on the Germans at every opportunity but failing due to poor command decisions which left them stuck behind the U.S. infantry divisions for most of the pocket's reduction. The performance of the U.S. 13th Armored Division was particularly disastrous. Two road marches totaling 400 kilometers sufficed for a Combat Command of the 13th Armored to decline to 50% of authorized strength for its Sherman tanks by the time it reached the battle area. Completely worn out, the division was immediately thrown into action on 10 April by XVII Airborne Corps commander Matthew Ridgway, who, under pressure from army commander Courtney H. Hodges to speed up operations, ordered it to encircle and "destroy" the German forces. The division commander, John B. Wogan, and his subordinates took this order literally.