Allied victory in World War I summer and autumn 1918: The Allied counteroffensive, known as the Hundred Days Offensive, began on 8 August 1918. The Battle of Amiens developed with III Corps Fourth British Army on the left, the First French Army on the right, and the Australian and Canadian Corps spearheading the offensive in the centre through Harbonnières. It involved 414 tanks of the Mark IV and Mark V type, and 120,000 men. They advanced 12 kilometers (7 miles) into German-held territory in just seven hours. Erich Ludendorff referred to this day as the "Black Day of the German army".

The Australian-Canadian spearhead at Amiens, a battle that was the beginning of Germany’s downfall, helped pull the British armies to the north and the French armies to the south forward. While German resistance on the British Fourth Army front at Amiens stiffened, after an advance as far as 14 miles (23 km) and concluded the battle there, the French Third Army lengthened the Amiens front on 10 August, when it was thrown in on the right of the French First Army, and advanced 4 miles (6 km) liberating Lassigny in fighting which lasted until 16 August. South of the French Third Army, General Charles Mangin (The Butcher) drove his French Tenth Army forward at Soissons on 20 August to capture eight thousand prisoners, two hundred guns and the Aisne heights overlooking and menacing the German position north of the Vesle. Another "Black day" as described by Erich Ludendorff.

Meanwhile General Byng of the Third British Army, reporting that the enemy on his front was thinning in a limited withdrawal, was ordered to attack with 200 tanks toward Bapaume, opening the Battle of Albert, with the specific orders of "To break the enemy's front, in order to outflank the enemies present battle front" (opposite the British Fourth Army at Amiens). Allied leaders had now realized that to continue an attack after resistance had hardened was a waste of lives and it was better to turn a line than to try to roll over it. Attacks were being undertaken in quick order to take advantage of the successful advances on the flanks and then broken off when that attack lost its initial impetus.

The British Third Army's 15-mile (24 km) front north of Albert progressed after stalling for a day against the main resistance line to which the enemy had withdrawn. Rawlinson’s Fourth British Army was able to battle its left flank forward between Albert and the Somme straightening the line between the advanced positions of the Third Army and the Amiens front which resulted in recapturing Albert at the same time. On 26 August the British First Army on the left of the Third Army was drawn into the battle extending it northward to beyond Arras. The Canadian Corps already being back in the vanguard of the First Army fought their way from Arras eastward 5 miles (8 km) astride the heavily defended Arras-Cambrai before reaching the outer defences of the Hindenburg line, breaching them on the 28 and 29 August. Bapaume fell on the 29 August to the New Zealand Division of the Third Army and the Australians, still leading the advance of the Fourth Army, were again able to push forward at Amiens to take Peronne and Mont St. Quentin on 31 August. Further south the French First and Third Armies had slowly fought forward while the Tenth Army, who had by now crossed the Ailette and was east of the Chemin des Dames, was now near to the Alberich position of the Hindenburg line. During the last week of August the pressure along a 70-mile (113 km) front against the enemy was heavy and unrelenting. From German accounts, "Each day was spent in bloody fighting against an ever and again on-storming enemy, and nights passed without sleep in retirements to new lines." Even to the north in Flanders the British Second and Fifth Armies during August and September were able to make progress taking prisoners and positions that were previously denied them.

On 2 September the Canadian Corps outflanking of the Hindenburg line, with the breaching of the Wotan Position, made it possible for the Third Army to advance and sent repercussions all along the Western Front. That same day Oberste Heeresleitung (OHL) had no choice but to issue orders to six armies for withdrawal back into the Hindenburg line in the south, behind the Canal Du Nord on the Canadian-First Army's front and back to a line east of the Lys in the north, giving up without a fight the salient seized in the previous April. According to Ludendorff “We had to admit the necessity to withdraw the entire front from the Scarpe to the Vesle.”

In nearly four weeks of fighting since 8 August, over 100,000 German prisoners were taken, 75,000 by the BEF and the rest by the French. Since "The Black Day of the German Army" the German High Command realized the war was lost and made attempts for a satisfactory end. The day after the battle Ludenforff told Colonel Mertz "We cannot win the war any more, but we must not lose it either." On 11 August he offered his resignation to the Kaiser, who refused it and replied, "I see that we must strike a balance. We have nearly reached the limit of our powers of resistance. The war must be ended." On 13 August at Spa, Hindenburg, Ludendorff, Chancellor and Foreign Minister Hintz agreed that the war could not be ended militarily and on the following day the German Crown Council decided victory in the field was now most improbable. Austria and Hungary warned that they could only continue the war until December and Ludendorff recommended immediate peace negotiations, to which the Kaiser responded by instructing Hintz to seek the Queen of Holland's mediation. Prince Rupprecht warned Prince Max of Baden "Our military situation has deteriorated so rapidly that I no longer believe we can hold out over the winter; it is even possible that a catastrophe will come earlier." On 10 September Hindenburg urged peace moves to Emperor Charles of Austria and Germany appealed to Holland for mediation. On the 14 September Austria sent a note to all belligerents and neutrals suggesting a meeting for peace talks on neutral soil and on 15 September Germany made a peace offer to Belgium. Both peace offers were rejected and on 24 September OHL informed the leaders in Berlin that armistice talks were inevitable.

September saw the Germans continuing to fight strong rear guard actions and launching numerous counter attacks on lost positions, with only a few succeeding and then only temporarily. Contested towns, villages, heights and trenches in the screening positions and outposts of the Hindenburg Line continued to fall to the Allies, with the BEF alone taking 30,441 prisoners in the last week of September. Further small advances eastward would follow the Third Army victory at Ivincourt on 12 September, the Fourth Armies at Epheny on 18 September and the French gain of Essigny-le-Grand a day later. On 24 September a final assault by both the British and French on a 4 mile (6 km) front would come within 2 miles (3 km) of St. Quentin. With the outposts and preliminary defensive lines of the Siegfried and Alberich Positions eliminated the Germans were now completely back in the Hindenburg line. With the Wotan position of that line already breached and the Siegfried position in danger of being turned from the north the time had now come for an assault on the whole length of the line.

The Allied attack on the Hindenburg Line began on 26 September including U.S. soldiers. The still-green American troops suffered problems coping with supply trains for large units on a difficult landscape. The following week cooperating French and American units broke through in Champagne at the Battle of Blanc Mont Ridge, forcing the Germans off the commanding heights, and closing towards the Belgian frontier. The last Belgian town to be liberated before the armistice was Ghent, which the Germans held as a pivot until Allied artillery was brought up. The German army had to shorten its front and use the Dutch frontier as an anchor to fight rear-guard actions.

When Bulgaria signed a separate armistice on 29 September, the Allies gained control of Serbia and Greece. Ludendorff, having been under great stress for months, suffered something similar to a breakdown. It was evident that Germany could no longer mount a successful defence.

Meanwhile, news of Germany's impending military defeat spread throughout the German armed forces. The threat of mutiny was rife. Admiral Reinhard Scheer and Ludendorff decided to launch a last attempt to restore the "valour" of the German Navy. Knowing the government of Prince Maximilian of Baden would veto any such action, Ludendorff decided not to inform him. Nonetheless, word of the impending assault reached sailors at Kiel. Many rebelled and were arrested, refusing to be part of a naval offensive which they believed to be suicidal. Ludendorff took the blame—the Kaiser dismissed him on 26 October. The collapse of the Balkans meant that Germany was about to lose its main supplies of oil and food. The reserves had been used up, but U.S. troops kept arriving at the rate of 10,000 per day.

Having suffered over 6 million casualties, Germany moved toward peace. Prince Maximilian of Baden took charge of a new government as Chancellor of Germany to negotiate with the Allies. Telegraphic negotiations with President Wilson began immediately, in the vain hope that better terms would be offered than by the British and French. Instead Wilson demanded the abdication of the Kaiser. There was no resistance when the social democrat Philipp Scheidemann on 9 November declared Germany to be a republic. Imperial Germany was dead; a new Germany had been born: the Weimar Republic.