In the spring of 1918 the German army attempted a series of war winning offensives on the Western Front that ultimately were to fail and their failure led directly to the German signing of an armistice in November of 1918. The failure of the Ludendorff offensives as they were known was strategic and operational in nature. The German army had devised a new tactical system and doctrine that broke the stalemate of the Western front. What they could not do was follow through once the front had been broken.
The Germans had developed the tactical system known as infiltration in response to the stalemate of trench warfare. This system concentrated on short sharp artillery bombardments followed by small groups of infantry attacking and infiltrating around points of resistance in order to maintain the momentum of the attack. This system worked admirably in piercing the defenses of the allies. By concentrating and achieving local superiority and tactical surprise, the Germans were able to break through the trench lines into open country. They proved this in the four offensives launched during the spring of 1918.
The problem was that after the front lines had been pierced and maneuver room gained the Germans were unable to capitalize on these gains. They faced a severe shortage of manpower and more importantly, of transport. The German army was not able to mass pursuit forces behind the attackers, due to requirements in the East and elsewhere on the Western Front. If the imperial army had had the troops available to exploit their gains, they still would have faced the obstacle of how to get them to the battlefield. Manpower considerations caused the German army to stagger the opening of their offensives with allowed these to concentrate their defensive forces to limit German gains. Additionally, the German army was not motorized to near the extent necessary to effect a pursuit once breakthrough was achieved, to say nothing of the difficulties involved with crossing the old trench-lines. The German lack of manpower and logistical difficulties eventually caused the offensives to run out of steam and the fighting then bogged down into positional warfare.
Even though the German army managed to mass almost 4,000,000 troops on the Western Front, they still did not outnumber their opponents. With the addition of America into the war in 1917, the German army had no choice but to attempt to win the war in early 1918, before American numbers began to be felt in the fighting. The fighting also cost the German army dearly, as they suffered around 500,000 casualties in the spring. These losses could not be mad good and the Germans were thus left defending a longer line with fewer troops than before the offensives.
The Ludendorff offensives were a grand failure, while the tactical system the Germans employed was superb the strategic and operational concept was flawed. Ludendorff violated the principles of mass, concentration at the point of attack, and operational unity. The offensives were misguided from the outset and the German army would have been better served if they had instead stood on the defensive while the political leadership sued for peace.
Chickering, Roger. Imperial Germany and the Great War, 1914-1918. Cambridge, UK: Cambridge University Press, 1998, 2004. Second edition, paperback. pp. 175-180
Keegan , John. The First World War, New York, NY: Random House, 1998. pp. 309-364
Strachan, Hew. The First World War, New York, NY: Penguin, 2003. pp. 267-301