Parallel to First Army's advance through the Hürtgen Forest, Ninth Army had to advance through the Rur plains. This terrain was fundamentally different from the dense forest, consisting of flat farmland with small villages. Planning for this area for both sides was different, as the Germans expected the Allied main thrust through this area, while it was actually through the Hürtgen Forest. One of the reasons for this decision was the dangerous Geilenkirchen-Salient at Ninth Army's northern flank, which would have threatened the American advance. This salient was reduced and rendered harmless in Operation Clipper, by a combined US-British attack until 22 November. The 84th Division of XIII Corps of the Ninth Army played a major role in this operation.