The Battle of the Bulge is a major battle fought in the densely forrested region called the Ardennes. The Allied Forces had pierced parts of the Westwall and where pushing to capture the German city of Aachen. But then the German army launched a counteroffensive, which was a surprise for the Allied units situated in the eastern part of Belgium.
The Battle of the Bulge was the bloodiest of the battles that U.S. forces experienced in World War II. Of course the German army paid a very high price as well, also the Belgian population suffered heavily, entire villages have been destroyed. So not surprisingly in the region of the battlefield many war memorials and war museums can be found.
This website is to remember this battle, it also offers a more information and points to other sources.