Which country joined the Allies in 1917, significantly influencing the outcome of the war?

The United States

The entry of the United States into World War I in April 1917 marked a decisive turning point, not just militarily, but also morally and economically. The American decision to join the fighting was largely influenced by the interception of the Zimmermann Telegram, in which Germany promised Mexico the American territories it had lost in exchange for its support against the United States. This event, combined with the relentless attacks by German submarines on civilian ships, galvanized American public opinion and led to direct engagement in the conflict. The arrival of fresh and well-equipped American troops bolstered the morale of the exhausted Allies and provided crucial support, both financially and militarily, hastening the defeat of the Central Powers.