What impact did the u. s. joining the triple entente have on the war?

Tipped the balance of the war in favor of the Triple Entente--The US joined in 1917 and World War I ended in 1918. 

The US though neutral in theory, had money and a vested interest in World War I. The government was leaning toward the Triple Entente and with Russian withdrawal in 1917 coupled with the Zimmerman Telegram, the US joined the war. Germany was able to move focus to the Western Front with the Russian withdrawal and that side was struggling to hold on. The US joining provided more weapons, fresh soldiers, and a new fire under the Western Front. Their efforts will help to push Germany back out of France and a surrender about a year later.