According to Wikipedia (... I know):
"The Battle of the Bulge was the bloodiest of the battles that U.S. forces experienced in World War II ... The German losses in the battle were critical in several respects: the last of the German reserves were now gone, the Luftwaffe had been shattered and the remaining German forces in the West were being pushed back."
It's also the source of one of the best comebacks ever recorded.
Here is what my history textbook has to say the Battle of the Bulge: