US involvement certainly shortened the war though not because they played a particularly important strategic role but rather because their entry into the war prompted Germany to launch a series of massive though rushed offensives with vague objectives.

The US entry into the First World War in 1917 coincided with the withdrawal of Russia from the conflict. This freed up more then a million German troops from the German Eastern Front for redeployment to the Western Front, dangerously tipping the balance of manpower in favour of the Germans. These were also veteran forces that had not experienced the level of attrition seen by those stationed on the Western Front. While it would be almost a year before those forces arrived in sufficient number and organisation to launch a major offensive, the German High Command were acutely aware that the arrival of American forces in strength was imminent and that something had to be done to win the war before the US arrivals could tip the balance of power back in the Entente's favour.

This resulted in the series of German offensive operations known collectively as the German Spring Offensive. Operation Michael was the main effort, directed at the British across the old Somme battlefield with three other operations, Georgette, Gneisenau and Blücher-Yorck being launched at different points along the Western Front. The German objective was to utilise the 50 divisions made available by the withdrawal of the Russians to win a decisive victory against the British and French before the US could arrive in force. The objectives of the operations however were rather vague and poorly defined, ultimately resulting in the squandering of German manpower despite their huge territorial gains. The loss of manpower, materiel and the impact on morale left the Germans open to a renewed British and French offensive spirit with culminated in the Hundred Days Offensive and led to the armistice in november 1918. The US was involved in the Hundred Days, operating alongside the French and undertook some major offensive actions such as the Meuse-Argonne Offensive.

The military contribution of the United States is a source of contention among some historians. When the US arrived, they failed to heed the warnings of French and British officers who tried to inform the US contingents of how the war should be fought. Quite a bit of this blame should fall on the US commander, General John Pershing who insisted on fighting the war his way. This led to the use of tactics that the British and French had discarded in 1914 due to their unsuitability for trench warfare. Such hubris led to high US casualties in the initial phases of US combat involvement. The US did adapt however and played an important but not strategically vital role in bringing Germany to the negotiating table.

Sources: The First World War by John Keegan Battle Tactics of the Western Front by Paddy Griffith

Doughboy War: The American Expeditionary Force in World War I by James Hallas