It Was About Bodies
Manpower was the big issue that shaped the campaign of 1918 and, ultimately, the end of the war before years' end. Consider this chart in which all the figures up through the Armistice are the actual numbers.
By late 1917 the war of attrition was working. All the countries in the war since 1914 were running out of bodies. The flood of American troops projected to arrive in 1918 and 1919 exercised a hypnotic power over both sides. The French were going to wait for the Americans, and late in the year event General Haig decided it would be imprudent to restart the Flanders offensive before the reinforcements arrived.
On the other side, Hindenburg and Ludendorff, whose staff had made similar projections, faced a dilemma after what seemed to them to be a pretty successful 1917 — Russia was out of the war, Italy very nearly the same after the Caporetto disaster, and they had held off the attacks in Flanders. However, they realized that they faced the same demographic issues as their long-time opponents. Eventually, despite the infusion of troops made possible by the ending of the war on the Eastern Front, they were inevitably going to be badly outnumbered on the Western Front. They faced three options:
1. Use the troops available from Russia to dig in and wait for the Allies to attack them. This, they viewed as just postponing the inevitable.
2. Negotiate a settlement. This option meant they would have to give up most of the occupied territory in western Europe. The two Crown Princes who commanded armies, Wilhelm and Rupprecht, supported this, but the generals found it unpalatable to turn things over to the diplomats.
3. Go on the offensive and try to solve things before the Yanks arrived in force. This is the choice they made, and you know what ensued.