In WW1, the USA managed to be pivotal to the defeat of Germany whilst hardly doing any fighting.
There is a view that when the Doughboys were shipped en masse across the Atlantic, Germany was either beaten in combat by massed US units or were so cowed by the huge forces being arrayed against them that they sued for peace. The reality is somewhat more subtle.
For example, the reason why Germany looked for a way out was because her economy was broken, her population was suffering terribly from food shortages and her armies had been soundly beaten by the British Empire and French forces. They were reeling back, demoralised and broken after suffering a shattering series of defeats (https://en.wikipedia.org/wiki/Hu...). US participation in these battles was very helpful but not decisive (see, for example: https://en.wikipedia.org/wiki/Me...) - the contribution made by the US units which were used to plug gaps in the Allied lines during the earlier German Spring Offensive was arguably more important from a military perspective.
To see the real US contribution, you need to look on a strategic level. Germany saw Britain and the Empire as the lynchpin of the Allies. Britain's navy blockaded Germany, her Merchant Navy transported goods all over the world and her economic power bankrolled the Allies. (https://en.wikipedia.org/wiki/Ec...) Germany felt she could not be victorious in Western Europe unless Britain was removed. In 1917, Hindenberg and his Chief-of-Staff Ludendorff took over the running of the German campaign, becoming de facto military dictators. They then made a risky decision. They would strangle Britain and force her out of the war by using unrestricted submarine warfare. They knew that this would inevitably bring the US into the fray and once this happened, there was maybe a year or two before US power became so great that Germany was bound to be defeated, but they thought that Britain could be forced to sue for peace after six months. Germany would then consolidate her gains in Belgium and France and seek an end to hostilities on her terms. Germany would be the outright victor, France the vanquished foe and Britain an impotent bystander. To do this, Germany undertook a massive military production programme, devastating the German economy in order to build up forces for a war-winning offensive. (https://en.wikipedia.org/wiki/Hi...)
What happened in reality was that the U-boat campaign failed when the convoy system was introduced, Britain was not appreciably weakened and Germany finished 1917 no closer to victory in the West. However, she had forced Russia out of the war, freeing up many divisions which she could move westward. Hindenburg and Ludendorff now took one last great gamble on winning the war before American troops could arrive in overwhelming force. They threw all of Germany’s strength into one massive blow, designed to split the Allies and force Britain to withdraw her troops from the continent. The subsequent Spring Offensive achieved great gains but ran out of steam and cost Germany so many casualties amongst her best soldiers that she was unable to resist the Allied riposte. After all these set-backs, Germany knew she was beaten and requested negotiations to end hostilities. She soon accepted the terms proposed by the Allies and signed to the Armistice on 11th November 1918.
