The Allied counteroffensive, known as the Hundred Days Offensive, began on 8 August 1918, with the Battle of Amiens. The battle involved over 400 tanks and 120,000 British, Dominion, and French troops, and by the end of its first day a gap 24 kilometres  long had been created in the German lines. The defenders displayed a marked collapse in morale, causing Ludendorff to refer to this day as the "Black Day of the German army". After an advance as far as 23 kilometres , German resistance stiffened, and the battle was concluded on 12 August. Rather than continuing the Amiens battle past the point of initial success, as had been done so many times in the past, the Allies shifted attention elsewhere. Allied leaders had now realised 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. They began to undertake attacks in quick order to take advantage of successful advances on the flanks, then broke them off when each attack lost its initial impetus.
How many days did the Battle of Amiens last?
4