The Entente war effort would have ground to a halt as it was in 1917 without the extension of American credit from the US entry into the war.
[T]he Foreign Office called together an interdepartmental committee on 30 September to consider how far Britain was dependent on the United States; the statements of the various departments were printed for the Cabinet on 6 November, and the conclusions were alarming. The Ministry of Munitions procured a large percentage of its guns, shells, metals, explosives and machine tools from the United States; the Army Department considered that there was no substitute for American supplies of oils and petroleum, nor for that of preserved meat; the Board of Trade stated that for cotton, for foodstuffs, for military necessities and for raw materials for industry, the United States was 'an absolutely irreplaceable source of supply'; the Board of Agriculture emphasised the dependence of Britain on the United States for grains; and finally, the Treasury stated baldly that 'Of the £5,000,000 which the Treasury have to find daily for the prosecution of the war, about £2,000,000 has to be found in North America' and added that there was no prospect of any diminution without a radical change in the policies of the Allied War Departments. The Treasury expressed, in its conclusion, the only action possible for the government: 'the policy of this country towards the U.S.A. should be so directed as not only to avoid any form of reprisal or active irritation, but also to conciliate and to please'
(...)
Thus the point had been reached where Britain no longer had control over her external financial affairs, but was at the mercy of events and the American government. While Britain waited for the Administration to decide between peace and war, Lever cobbled up one expedient after another to stave off disaster, and Britain struggled on, sometimes with little more than a week's money in hand for American payments. By mid-March the Chancellor asked for an assessment of the position, and the conclusion was that the Treasury, with all expedients, could see little more than a month ahead(...)Finally, on 2 April 1917 the President asked Congress to recognise a state of war between the United States and Germany. That same day the Chancellor reported to the Cabinet on the financial situation. On 1 April Britain had $490m. worth of securities in the United States, but this was balanced by an overdraft of $358m.; there was $87m. in gold, making a net total of $219m. actually in New York, but expenditure was at the rate of $75m. a week, so it would only last three weeks. Beyond that, the only visible asset was £114m. in gold in the Bank of England and the Joint Stock Banks.
(Remember Britain was itself underwriting the war efforts of France, Italy and Russia with billions in credit)
Perhaps the Entente could have made up some of the shortfall in American supply by massively suppressing domestic consumption, but it would have required a full command economy risking runaway inflation, heavy rationing and food shortages and domestic unrest similar to the experience of Germany's home front, which did take this option. And even then it seems doubtful to me the Entente would be able to achieve the sort of massive advantage in men and materiel required to break through the Hindenburg Line and achieve a decisive victory on the Western Front. Much more likely there is a negotiated peace in 1917 or 1918 when it becomes apparent that decisive victory is not possible outside of a desperate gamble that either way would leave their economies and societies wrecked post-war.
