What if the USA had joined the Entente at the beginning of the war whilst the British Empire remained neutral at the beginning but eventually joined one of the sides towards the end of the war?
Well, I don't think that a war would have even broken out in the West if such things ever happened, unless the German leaders go completely brainless.“If I had been President,” Theodore Roosevelt wrote to the British ambassador to the United States, Cecil Spring-Rice, on October 3, 1914, “I should have acted on the thirtieth or thirty-first of July, as head of a signatory power of the Hague treaties, calling attention to the guaranty of Belgium’s neutrality and saying that I accepted the treaties as imposing a serious obligation which I expected not only the United States but all other neutral nations to join in enforcing.” Simeon Strunsky, “Theodore Roosevelt and the Prelude to 1914,”
Never underestimate the capacity for German brainlessness...... unless the German leaders go completely brainless.