The role of America was absolutely decisive in the Great War, for two reasons. One involved our participation in the war, but the second was actually more important and far-reaching.
The armies on the Western Front were exhausted after four years of war, and after the mutinies of 1917 the French Army was almost useless. The Treaty of Brest-Litovsk would allow the Germans to transfer troops to the West, and even if they failed to defeat the British they could have forced a negotiated peace upon the Entente (Allies). The influx of American troops, not only as reinforcements to their allies but organized into their own national army and taking over sectors of the front, changed the balance of power. The masses of troops armed with superior weapons gave a morale boost to the Allies, as well as simply overwhelming the depleted German armies.
But what really won the war was the British blockade of Europe, which strangled Germany materially and economically. This blockade was established in the first weeks of the war, and could never have worked without the support of the US.
The Brits had degrees of contraband, including Absolute Contraband, which meant war materials. But there were many products which were termed war materiel, including cloth, medicines, raw materials, foods, anything really. And the British applied what was termed the Doctrine of Continuous Voyage. This meant that goods shipped even from a neutral country to another neutral (say, America to Sweden) would be considered contraband if the British believed it might eventually be shipped on to Germany. The British needed no proof to board a ship and confiscate the cargo.
The British were very worried about this, as this was the very doctrine which we went to war over in 1812, when they were blockading Napoleonic Europe. Fortunately for the Allies, President Wilson believed Prussian militarism was more dangerous to world freedom than British arrogance.
WWI occured as a result of political instability, alliances, arms build-up, mistrust, etc. Much simplified, here is what happened:
1. Archduke Ferdinand of Austria-Hungary was killed and Austria-Hungary believed that Serbia had something to do with it, so they wanted to crush Serbia.
2. Germany was an ally of Austria-Hungary, and promised to take their side and help them if they needed it.
3. Russia was an ally of Serbia, so they declared war on Austria-Hungary, so Germany declared war on Russia.
4. Germany knew France would not stay out of a war between Germany and Russia, so Germany declared war on France also.
5. Germany wanted to surround France, but to do so they had to go through Belgium. They asked permission and Belgium said no, so Germany declared war on Belgium too.
6. Great Britain was an ally of Belgium, so they declared war on Germany. And so began WWI. America was not involved at this point.
The war seemed like it would last forever. Neither side could force a decisive victory and neither seemed so superior that the other side was tempted to give up. When, at last, peace talks were held, the United States was involved and was a major player in events outside of itself for the first time in history.
President Wilson wanted the peace to be fair to Germany, while the Allies blamed Germany for what happened. Wilson gave a speech where he outlined the Fourteen Points that he thought would bring about a fair peace.