The US entry shortened the war, but IMO the Entente didn't even need the USA to win the war. Germany was already starting to starve in 1917 and their last big offensive (Kaiserschlacht) was against French and Commonwealth troops.
If the Americans had not joined the war when they did, the British would have run out of American currency in about April or May 1917. Unable to purchase the relevant raw materials (and some finished products) from the United States, they would have been compelled to seek alternatives elsewhere, among the Empire, in Argentina, and so on - alternatives that they were already pursuing to the best of their ability anyway. It's not clear whether the British would run out of food or military supplies first, but it is clear that the army on the Continent would be quickly immobilized, that riotous conditions would have developed in Britain itself, that industry, unable to supply all of its needs with American raw materials, would have ground to a halt, with concomitant increases in social conflict, and that even Lloyd George would have found it difficult to support a continuation of the war under such circumstances.
Furthermore, the success of British finances during the war buttressed that of France; if the British either stopped buying in America or were forced to stop buying by the American Federal Reserve (something that briefly happened in 1916 as it was; the British desperately tried to shore up supplies by buying elsewhere in the Americas, but it wasn't enough, while the City exchanges took a huge hit - of course there was a run on the pound as well - the Americans reversed their stance quickly on the grounds that the British weren't totally overleveraged in the US yet, but by March 1917, the British situation with regard to the amount of dollars on hand had gotten considerably worse, not better), the French would have taken an even bigger hit. French borrowing was sustained by the impression that British industry could fuel the Entente war effort, and sometimes even by direct British financial support. If it was made inescapably clear that the British could not, in fact, fuel the war effort, the French economy would collapse right along with that of Britain.
Hence the resumption of unrestricted U-boat warfare was precisely the worst
possible policy for Germany in the spring of 1917; not only did the counter-blockade fail, but the British were rescued from virtually certain economic collapse by the entry of the United States into the war, which shored up British finances and made victory possible again.
Or, to put it another way: Erich Ludendorff was the Tony Romo of World War I.
