Wikipedia:
The United States of America declared war on Germany in 1917 on the grounds that Germany had violated U.S. neutrality by attacking international shipping and because of the Zimmermann Telegram sent to Mexico.[4] The U.S. entered the war as an "associated power", rather than as a formal ally of France and the United Kingdom, in order to avoid "foreign entanglements".[5] Although the Ottoman Empire and Bulgaria severed relations with the United States, neither declared war on her.[6]

Nearly one million French soldiers (306,000 in 1914; 334,000 in 1915; 217,000 in 1916; 121,000 in early 1917,) out of a population of twenty million French males of all ages, had been killed in fighting by early 1917. Just after the disastrous Nivelle offensive at the Aisne, mutinies in over half of the French regiments on the Western Front. The new commander General Philippe Pétain restored morale by talking to the men, promising no more suicidal attacks, providing rest for exhausted units, home furloughs, and moderate discipline. But this process took time.

After the October Revolution, Russia left the alliance and ended formal involvement in the war, by the signing of the treaty of Brest Litovsk in November effectively creating a separate peace with the Central Powers. This was followed by Romanian cessation of hostilities, however the Balkan State declared war on Central Powers again on 10 November 1918.

In April 1918 operational control of all Entente forces on the Western Front passed to the new supreme commander, Ferdinand Foch.

The Russian withdrawal allowed for the final structure of the alliance, which was based on five Great Powers:

• French Republic
• British Empire
• United States
• Italy
• Japan