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 skillfully 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.