Reich Foreign Secretary Zimmermann's telegram to Mexico urging her entry into war against the United States is discovered and translated by the British.

Photo: Detail of the Zimmermann Telegram

Germany again declares unrestricted submarine warfare.

Photo: German submarine

Tsar Nicholas II of Russia abdicates. Provisional government is declared.

President Wilson asks Congress for a declaration of war with Imperial Germany. (Link goes to PBS: American Experience)

Photo: President Wilson addressing Congress

French launch disastrous offensive at Chemin des Dames, advancing only 500 yards at the cost of 250,000 plus casualties.

Over half a million French soldiers mutiny, or "go on strike" and refuse to continue the failed offensive.

The United States passes the Selective Service Act empowering the Federal Government to draft men for the armed forces.

Photo: Selective Service goes into effect

Russian Army lead by Alexander Kerensky begins last -- and disastrous -- offensive in Galicia.

The first wave of the American Expeditionary Force lands in France.

Photo: George Patton with tank

Third Battle of Ypres, known as Passchendaele, results in minor gains, but still no breakthrough in the Western front at the cost of 700,000 casualties for both sides.

American troops in France fire their first shot in trench warfare.

Photo: American artillery gun, nicknamed Calamity Jane

Bolshevik socialists, led by Lenin, overthrow Kerensky's government.

Photo: A smiling Vladimir Lenin

The new Russian government, represented by Leon Trotsky, signs an armistice with Germany.

British capture Jerusalem from the Turks and her Arab allies.