Hundred Days Offensive

The Hundred Days Offensive (8 August to 11 November 1918) was a series of massive Allied offensives that ended the First World War. Beginning with the Battle of Amiens (8–12 August) on the Western Front, the Allies pushed the Imperial German Army back, undoing its gains from the German spring offensive (21 March – 18 July).

The Germans retreated to the Hindenburg Line, but the Allies broke through the line with a series of victories, starting with the Battle of St Quentin Canal on 29 September. The offensive led directly to the Armistice of 11 November 1918 which ended the war with an Allied victory. The term "Hundred Days Offensive" does not refer to a planned Allied campaign, but rather the rapid series of Allied victories.

The German spring offensive on the Western Front had begun on 21 March 1918 with Operation Michael and had petered out by July. The German Army had advanced to the River Marne, but failed to achieve their aim of a victory that would decide the war. When the German Operation Marne-Rheims ended in July, the Allied supreme commander, Ferdinand Foch, ordered a counter-offensive, which became known as the Second Battle of the Marne. The Germans, recognizing their untenable position, withdrew from the Marne to the north. For this victory, Foch was granted the title Marshal of France.

After the Germans had lost their forward momentum, Foch considered the time had arrived for the Allies to return to the offensive. General Pershing who commanded the American Expeditionary Force (AEF) was keen to use his army as an independent force. The British Expeditionary Force (BEF) had been reinforced by large numbers of troops returned from the Sinai and Palestine campaign and from the Italian front, and by replacements previously held back in Britain by Prime Minister David Lloyd George.

The military planners considered a number of proposals. Foch agreed to a proposal by Field Marshal Sir Douglas Haig, commander-in-chief of the BEF, to strike on the River Somme, east of Amiens and south-west of the site of the 1916 Battle of the Somme, to force the Germans away from the vital Amiens–Paris railway. The Somme was chosen because it remained the boundary between the BEF and the French armies, along the Amiens–Roye road, allowing the two armies to cooperate. The Picardy terrain provided a good surface for tanks, unlike in Flanders, and the defences of the German 2nd Army under General Georg von der Marwitz were relatively weak, having been subjected to continual raiding by the Australians in a process termed peaceful penetration.

The Battle of Amiens (with the French attack on the southern flank called the Battle of Montdidier) opened on 8 August, with an attack by more than 10 Allied divisions—Australian, Canadian, British, and French forces—with more than 500 tanks. The mastermind of the plan was the Australian Lieutenant General John Monash. Through careful preparation, the Allies achieved surprise. The attack, led by the British Fourth Army, broke through the German lines, and tanks attacked German rear positions, sowing panic and confusion. By the end of the day, a gap 15 mi (24 km) wide had been created in the German line south of the Somme. The Allies had taken 17,000 prisoners and 339 guns. Total German losses were estimated to be 30,000 men, while the Allies had suffered about 6,500 killed, wounded and missing. The collapse in German morale led Erich Ludendorff to dub it "the Black Day of the German Army".

The advance continued for three more days but without the spectacular results of 8 August, since the rapid advance outran the supporting artillery and ran short of supplies. During those three days, the Allies had managed to gain 12 mi (19 km). Most of this was taken on the first day as the arrival of German reinforcements after this slowed the Allied advance. On 10 August, the Germans began to pull out of the salient that they had managed to occupy during Operation Michael in March, back towards the Hindenburg Line.

Haig refused to continue the attack and instead prepared to launch a fresh offensive by the Third Army at Albert (the Battle of Albert), which opened on 21 August. The offensive was a success, pushing the German 2nd Army back over a 34 mi (55 km) front. Albert was captured on 22 August. The attack was widened on the south, by the French Tenth Army starting the Second Battle of Noyon on 17 August, capturing the town of Noyon on 29 August. On 26 August, to the north of the initial attack, the First Army widened the attack by another 7 mi (11 km) with the Second Battle of Arras of 1918. Bapaume fell on 29 August (during the Second Battle of Bapaume).