That's inaccurate to say the least. The final trigger for US entry into the war was the Zimmerman telegram and as far a unlimited submarine warfare goes the Germans had tried it previously and been warned off by the USA. Their decision to resume USW despite prior warnings primed things so when the telegram was intercepted a US declaration of war was all but inevitable. Much like WW2 it took time for the USA to gear up for war and the bulk of the fighting during the decisive battles were fought mainly by the French and British. If the war had lasted until 1919, as most people expected, then the US contribution would have been the decisive factor, though if the war went on that long I suspect the Entente would have fought their way to Berlin.