Battle of the Bulge

Often called Hitler's last gamble the Battle of the Bulge was one final attempt by the German Wehrmacht to try to turn the tide of the war. After D-Day and the eventual liberation of Paris, the Germans were in a really bad position. Not only were they now loosing in the east after the Sixth Army got destroyed in the disastrous Stalingrad offensive but now the Germans had to fight a two-front war against the Allied powers in the west and the Comintern in the east.
Hitler wanted to try and drive the allies out of the western front in order to focus on the Soviets and the eastern front. The allies were advancing quickly and their troops were poorly supplied due to long and underdeveloped supply chains and fatigued from battle. Hitler hoped to make a breakthrough in the Ardeness forest in order to split the Allied front in two and to try and capture the main supply port of Antwerp from the Allies. Hitler (perhaps extremely optimistically) thought that if he could do this then Germany might be able to sign a separate peace with the allies and focus on the Eastern Front (although even if the plan had been successful it's doubtful that the Allies would have signed a separate peace).
On December 16, 1944, the Wehrmacht threw just about everything they could at the allied forces in the Ardeness forest, their strongest infantry with armor and the element of surprise. (Fun fact this was the third time the Germans pushed through the Ardeness to try and outflank the Allies the other two times being the Schlieffen Plan and the Fall Gleb, you really would have thought that the Allies would've learned their lesson by now). Within a day a large pocket of allied troops was encircled in the Ardeness including the famous 101st airborne division. The cold weather, difficult terrain, element of surprise, and German deception tactics (sending English speaking germans in American uniforms to deceive the American army) lead to the allies initially being stunted. To make matters worse the Allies couldn't even use their air superiority because of the poor weather and difficulty targeting in a forest. When the Germans called for the Encircled Americans to surrender, General Anthony Clement McAuliffe famously denied the call saying, "nuts!". The Germans were unable to achieve the lightning victory they had hoped for being stalled in the Ardeness forest instead of being forced to use their dwindling resources in a long drawn-out battle that lasted until January 25. The US lost 89,000 men making it the second deadliest battle fought by US forces ever. In comparison, the Germans lost almost 100,000 men which unlike the Allied troops could not be replaced due to the German dwindling manpower supply. This last-ditch effort caused the utter collapse of the German Wehrmacht and the allies easily stormed across the Rhine and was considered the last real German attempt at success from there on out it was just a slow delay of the inevitable.

https://en.wikipedia.org/wiki/Battle_of_the_Bulge
https://en.wikipedia.org/wiki/Anthony_McAuliffe
https://www.youtube.com/watch?v=DZmPM9s1mTs