http://en.wikipedia.org/wiki/Battle_of_Hürtgen_Forest
Basically, a conglomerate of fierce german defenders hide in an impenetrable forest over the winter of 44 to 45. using the forest as a base.
The allies want to blow up a dam on the rhur river, and decide that going through the forest is the most direct route.
Air units can’t see a damn thing, and tanks can barely navigate the few roads that exist. Everything is mined/covered in barbed wire. anti-tank guns, pill boxes, and improvised booby traps are replete.
It was impossible to maintain an effective front-line, or be confident areas were -clear- of enemy activity.
The scale of the conflict is also HUGE…. 120,000 Yankee’s vs 80,000 Krauts…
They called it hell’s forest for a reason…