Battle of the Bulge was a Nazi offensive. Germans used to know this as "Operation Watch on the Rhine", while Allies on the other hand used to know this as Ardennes Counteroffensive.
The US troops were already bearing down on the Rhine and the desperate Germans decided to launch a counter attack to push back the Allies and create a bulge in the US front line that would (hopefully) force the Allies to retreat so the Germans could prepare defences. Germans massed all reserves, tanks and planes they had not trying to (also) desperately stop the Soviets and struck the Ardennes Forest in Belgium against the exhausted US 1st Army. Initially, the attack was a major success, and in three days the Germans succeeded in creating a bulge, albeit not as big as they aimed. The USAAF was also grounded due to snowstorms, ensuring German troops would not be bombed. They quickly gained over 80km of ground.
The Failure of Germany
However, just about every army in the area turned to the Bulge and launched a massive relief operation against the overstretched Germans. The weather cleared and the Allied Air Power Germany feared forced the Germans to retreat and the Bulge was filled back in by Allied soldiers. The Battle of the Bulge had been a major disaster, as 90000 men were either dead or captured and with the rest of the forces occupied with the Soviets, all the Germans could do in the Western Front was defend.