On June 28, 1914 the Great War (known mostly in the last 75 years as World War I) began as the Archduke Franz Ferdinand and his wife were assassinated in Sarajevo. The war ensued as decades and longer alliances lined up on either side with Germany, Austria-Hungary, the Ottoman Empire and Bulgaria on one side and most of the rest of the world on the other.
The Great War was a brutal and vicious war that saw the use of mechanization, airplanes, and chemical weapons on grand scale for the first time. It was a war with unspeakable casualties over pittances of battlefields. Often the two sides traded land back and forth several times at tremendous costs. It was not until the US joined the fight much later, was anything in a way of advantage for either side realized.
And it was the conclusion of the Great War that laid the ground for any number of wars yet to be fought. The War to End All Wars was a misnomer of epic proportion. The seeds of World War II (in both Europe and the Pacific), much of the turmoil in the Middle East, and virtually all the conflicts in what was once colonial Africa are directly attributable to the methods by which the Great Powers chose to reshape the world.