Between 16 December 1944 and 23 February 1945, the U.S. Ninth Army was unable to advance across the Rur during Operation Queen, because German forces controlled dams close to the river's source in the densely forested region of the Hohes Venn. This meant Axis forces could potentially blow the dams, releasing enough water to wash out an Allied assault. At the same time, the German Ardennes Offensive meant any further westward push would leave Allied forces stretched and supply lines exposed. Eventually the counteroffensive was overwhelmed and German engineers, under pressure of aerial and artillery bombardment, released the dams. As the flood waters began to subside, Allied forces crossed the Rur in rafts in the early hours of February 23, 1945, as part of Operation Grenade.