What happened on march 15 1917?

Answer 1

The Germans launched Operation Michael (commonly known as the German Spring Offensive of 1918).

Operation Michael introduced new methods of warfare, and were the testing grounds for the Storm Trooper (Stoßstruppen), men that were equipped and trained specifically to break through the weak points in enemy defenses and be able to weaken trenches and their support significantly, allowing the armies that followed them to defeat the trench easily; breaking the deadlock on the Western Front.

In the offensive and battles that followed, the Germans managed to make massive (in first world war terms) territorial gains that had not been seen since the opening days of 1914. Eventually the offensive outstretched itself and petered out, and was finally reversed during the Battle of Amiens.

The Spring Offensive was essentially Germany's last ditch effort to win the First World War, you could call it the Kaiser's Gambit; the German field armies were desperately trying to outrun defeat at home with victory on the battlefield, since public support for the war was waning very quickly. The Americans were also beginning to land in France, the manpower on the Entente side would soon stack up significantly against the Central Powers, and if the opportunity was not siezed upon, it was thought, the Germans would have no chance to bring an end to the war.

Answer 2

That day Nicholai II was abdicated.