Nikolas II, Czar of Russia, declares war against Germany from the Winter Palace in St. Petersburg, 1914
Why War? The Christmas Truce Trench Warfare Soldiers at War (in progress)
World War I was usually called "The Great War" until World War II broke out less than 21 years after The Great War ended (or at least when The Great War ended on "The Western Front" - it continued in Eastern Europe for a few more years). The War began in the summer, 100 years ago, as a struggle between the Austro-Hungarian Empire and Serbia exploded into a multinational conflict because of interlocking "mutual defense" alliances between European powers. The events of summer 1914 began the bloody "historical 20th Century" (usually seen as 1914-1989, although Americans might see it as covering 1896-1989), in which Europe was in a state of almost continuous conflict.
The Great War Begins... (with Video)