The Battle of the Bulge, also known as the Ardennes Offensive, was the last major German offensive campaign on the Western Front during World War II. The battle lasted for five weeks from 16 December 1944 to 28 January 1945, towards the end of the war in Europe. It was launched through the densely forested Ardennes region between Belgium and Luxembourg. It overlapped with the Alsace Offensive, subsequently the Colmar Pocket, another series of battles launched by the Germans in support of the Ardennes thrust.

The primary military objectives were to deny further use of the Belgian Port of Antwerp to the Allies and to split the Allied lines, which potentially could have allowed the Germans to encircle and destroy the four Allied forces. The Nazi dictator Adolf Hitler, who since December 1941 had assumed direct command of the German army, believed that achieving these objectives would compel the Western Allies to accept a peace treaty in the Axis powers' favor. By this time, it was palpable to virtually the entire German leadership including Hitler himself that they had no realistic hope of repelling the imminent Soviet invasion of Germany unless the Wehrmacht was able to concentrate the entirety of its remaining forces on the Eastern Front, which in turn obviously required that hostilities on the Western and Italian Fronts be terminated. The Battle of the Bulge remains among the most important battles of the war, as it marked the last major offensive attempted by the Axis Powers on the Western front. After their defeat, Germany would retreat for the remainder of the war.

The Germans achieved a total surprise attack on the morning of 16 December 1944, due to a combination of Allied overconfidence, preoccupation with Allied offensive plans, and poor aerial reconnaissance due to bad weather. American forces bore the brunt of the attack. The Germans attacked a weakly defended section of the Allied line, taking advantage of heavily overcast weather conditions that grounded the Allies' superior air forces. Fierce American resistance on the northern shoulder of the offensive, around Elsenborn Ridge, and in the south, around Bastogne, blocked German access to key roads to the northwest and west that they counted on for success. Columns of armor and infantry that were supposed to advance along parallel routes found themselves on the same roads. This congestion, and terrain that favored the defenders, threw the German advance behind schedule and allowed the Allies to reinforce the thinly placed troops.

The farthest west the offensive reached was the village of Foy-Nôtre-Dame, south east of Dinant, being stopped by the U.S. 2nd Armored Division on 24 December 1944. Improved weather conditions from around 24 December permitted air attacks on German forces and supply lines, which sealed the failure of the offensive. On 26 December the lead element of Patton's U.S. Third Army reached Bastogne from the south, ending the siege. Although the offensive was effectively broken by 27 December, when the trapped units of 2nd Panzer Division made two break-out attempts with only partial success, the battle continued for another month before the front line was effectively restored to its position prior to the attack. In the wake of the defeat, many experienced German units were out of men and equipment, and the survivors retreated to the Siegfried Line.

The Germans' initial attack involved around 410,000 men; just over 1,400 tanks, tank destroyers, and assault guns; 2,600 artillery pieces; and over 1,000 combat aircraft, as well as large numbers of other armored fighting vehicles (AFVs). These were reinforced a couple of weeks later, bringing the offensive's total strength to around 450,000 troops, and 1,500 tanks and assault guns. Between 63,222 and 98,000 of these men were killed, missing, wounded in action, or captured. The battle severely depleted Germany's armored forces, which remained largely unreplaced throughout the remainder of the war. German Luftwaffe personnel, and later also Luftwaffe aircraft (in the concluding stages of the engagement) also sustained heavy losses.

From among the Americans' peak strength of 610,000 troops, there were 89,000 casualties, including about 19,000 killed. The "Bulge" was the largest and bloodiest single battle fought by the United States in World War II and the third-deadliest campaign in American history.