1. Von Falkenhayn completes preparation for great Austro-German campaign against Russia under Von Mackensen, assembling 26 army corps and over 4000 guns.
American steamer "Gulflight" attacked by German submarine; 3 American lives lost.
2. Austro-Germans begin a general offensive in Galicia. Von Mackensen inflicts disastrous defeat on Russians at Gorlice.
4. Italy renounces the Triple Alliance.
7. British passenger steamship "Lusitania" is sunk without warning by German submarine off Irish coast, with a loss of 1152 lives including 114 Americans, among them Elbert Hubbard, Albert G. Vanderbilt, Charles Frohman, and J. M. Forman.
8. Germans, invading Courland, enter Libau.
9. General d'Urbal, assisted by Foch, Joffre, and later by Pétain, with seven army corps and 1100 guns, begins the great Battle of Artois.
10. Repeated attacks by Anzac forces fail to capture Turkish positions at Gallipoli. Russian offensive gains against the Austrians.
11. French capture Notre Dame de Lorette and Carency, in the Artois sector.
13. President Wilson sends a note calling upon the German government to disavow the illegal sinking of the "Lusitania" and other ships.
15-17. Russians severely defeated on the San.
17. French capture the left bank of the Yser-Ypres canal, virtually ending the Second Battle of Ypres which, despite local successes due to the use of poisonous gas, resulted in German defeat.
20. British end 12-day attempt to carry Aubers Ridge with the view of retaking Lille. The net result of the battles of Aubers Ridge and Festubert was an advance of 600 yards on a front of 4 miles.
23. Italy declares war on Austria-Hungary to take effect May 24.
24. San Marino declares war against Austria.
27. Italians cross the Isonzo river near Monfalcone and capture Pilcante and Ala.
Russians force Germans at Sieniawa to retreat across the San with heavy loss.
28. British and French capture important Turkish positions on Gallipoli peninsula.
Austrian aeroplanes bombard Venice.
Germany makes a noncommittal reply to President Wilson's demands concerning submarine warfare.
31. French capture Souchez, one of the last important actions in the Battle of Artois.