A prolonged conflict, a modified Treaty of Versailles and no League of Nations may have transpired without direct American involvement in WWI

US General John J Pershing arrives in France, 3 June 1917

A lthough the United States had supported the Allied cause in the First World War with weapons, supplies and financial considerations, the introduction of American troops and other military assets in a direct role in 1917-18 was welcomed by the war-weary nations of Britain and France, and was an ominous development for Germany and the Central Powers. However, the absence of this US commitment to an active military role in the Great War may well have altered the outcome of the conflict along with the structure and steadiness of the tenuous peace that followed the armistice and the terms of the Treaty of Versailles.

How would the outcome of the Great War have been altered if the US had not entered the fight?

Clive Webb: Historians are not usually in the prediction business, not least when it concerns a future that never came to pass, and that may account for the lack of consensus about how US neutrality would have affected the outcome of the war. Some alternative histories have the Central Powers claiming victory. That would have led to Germany dictating peace terms that established it as the hegemonic power in Europe. Britain and France would’ve been ruined economically and unable to pay their debts, which in turn would’ve had ruinous consequences for their US creditors.

Others insist that the Allies would eventually have won. At least one historian has claimed that it would have been better for the USA not to intervene because the mobilisation of the American Expeditionary Forces (AEF) expended time, effort and resources that should have been invested in the British and French military. The USA therefore slowed rather than accelerated Allied victory. In a history where Britain and France used their unrestricted access to US war production to overcome the Central Powers, they would have no need to invite US President Woodrow Wilson to the peace table and could have imposed more punitive terms on their defeated foe. That settlement would have restored the same international system that had led to war in the first place.