By the 25th of September the German armies were back on the great line devised by Hindenburg in the autumn of 1916. The one chance left to them was to hold out there during the winter, in the hope that they might be able to bargain with the Allies. If the Allies attacked, there were two sections which Ludendorff viewed with anxiety. One was his left wing on the Meuse, where, if the Allies broke through, the Hindenburg Line would be turned on its flank. The other was the German centre from Douai to St. Quentin, the main Hindenburg Line, which was not only the fortress where he hoped to pass the winter, but the one protection of the great railway from Lille by Valenciennes to Mezières, on which his whole position depended. He therefore laboured to keep his left and centre as strong as possible; for, in spite of his experience in August and September, he could not conceive the possibility of an assault on every section.
For Foch this was to be the crowning battle of the war. If he could break through the German centre, and at the same time turn the German left, defeat would stare the enemy in the face, and there would be victory long before Christmas. If the Americans on the Meuse succeeded, they would make retreat imperative; but if Haig in the centre succeeded, he would make retreat impossible, and disaster must follow. The British were assigned the most difficult part. They had to attack in the area where the enemy defences were most highly organized and his forces strongest. If the Hindenburg Line held, the German courage might yet recover, and a new era of resistance begin. Haig's armies had already borne the heaviest share of the summer fighting, and every division had been sorely tried. Yet the attempt must be made, for it was the essential part of the whole strategy, and the measure of difficulty was the measure of the honour in which Foch held the fighting qualities of his British allies.
In deciding to make the attack, and to break the Hindenburg Line at one blow, Sir Douglas Haig stood alone. So difficult seemed the operation that the British Government were in the gravest doubts, and left the burden of responsibility upon the Commander-in-Chief. Even the French generals hesitated. The movement was undertaken on Sir Douglas Haig's initiative; he bore the whole burden of it; and therefore to him belongs the main credit of what was destined to be one of the decisive actions of the war.