April 1917

April 2 – WWI: U.S. President Woodrow Wilson asks the United States Congress for a declaration of war on Germany.

April 6 – WWI: The United States declares war on Germany.

April 8 (N.S.) (March 26, O.S.) – In Petrograd, 40,000 ethnic Estonians demand national autonomy within Russia.

April 9–12 – WWI: Canadian troops win the Battle of Vimy Ridge.

April 10 – An ammunition factory explosion in Chester, Pennsylvania kills 133.

April 12 (N.S.) (March 30 O.S.) – The Autonomous Governorate of Estonia is formed within Russia from the Governorate of Estonia and the Northern part of the Governorate of Livonia.

April 16  (N.S.) (April 3, O.S.) – Vladimir Lenin arrives at the Finland Station in Petrograd.
 
WWI: The Nivelle Offensive commences.

April 17  (N.S.) (April 4, O.S.) – Vladimir Lenin's April Theses are published. They become very influential in the following July Days and Bolshevik Revolution.

WWI: The Egyptian Expeditionary Force begins the Second Battle of Gaza. This unsuccessful frontal attack on strong Ottoman defences along with the first battle, resulted in 10,000 casualties, the dismissal of the force commander General Archibald Murray and the beginning of the Stalemate in Southern Palestine.

The Times and the Daily Mail (London newspapers both owned by Lord Northcliffe) print atrocity propaganda of the supposed existence of a German Corpse Factory.

April 26 – WWI: Agreement of Saint-Jean-de-Maurienne between France, Italy and the United Kingdom to settle interests in the Middle East signed.