May 1917

May 9 – WWI: The Nivelle Offensive is abandoned.

May 13 – The nuncio Eugenio Pacelli, the future Pope Pius XII, is consecrated Archbishop by Pope Benedict XV.

May 13–October 13 (at monthly intervals) – 10-year-old Lúcia Santos and her cousins Francisco and Jacinta Marto report experiencing a series of Marian apparitions near Fátima, Portugal, which becomes known as Our Lady of Fátima.

May 15 – Robert Nivelle is replaced as Commander-in-Chief of the French Army by Philippe Pétain.

May 18 – WWI: The Selective Service Act passes the United States Congress, giving the President the power of conscription.

May 21 – Over 300 acres (73 blocks) are destroyed in the Great Atlanta fire of 1917.

May 22 – Commissioned Officer Corps of the U.S. Coast and Geodetic Survey.

May 23 A month of civil violence in Milan, Italy, ends after the Italian army forcibly takes over the city from anarchists and anti-war revolutionaries. Fifty people are killed and 800 arrested.
 
WWI: During the Stalemate in Southern Palestine the Raid on the Beersheba to Hafir el Auja railway by Desert Column of British Empire troops, destroys large sections of the railway line linking Beersheba to the main Ottoman desert base.

May 26 – A tornado strikes Mattoon, Illinois, causing devastation and killing 101 people.

May 27 – WWI: Over 30,000 French troops refuse to go to the trenches at Missy-aux-Bois.

May 27 – Pope Benedict XV promulgates the 1917 Code of Canon Law