World War I, which lasted from 1914 to 1918, was the biggest and bloodiest conflict ever fought in human history. It introduced the world to the horrors of trench warfare and lethal new technologies like tanks, airplanes, poison gas, and flamethrowers. The fighting ultimately ended the long-held empires of Germany, Russia, and Austria-Hungary, while bringing into existence new nations and radically changing the map of Europe. It was also deeply unjust and strategically stupid, resulting in the worst loss of life in such a short time in all of history—until World War II outdid it two decades later.
By summer of 1914, Europe was in crisis. The assassination of Archduke Franz Ferdinand in Sarajevo had triggered an avalanche of war preparations that led to the first truly global war. No one expected the carnage that would follow, with millions of military and civilian lives lost over the course of four years.
The major belligerents were all locked into opposing alliances that ensured that any war between Russia and Austro-Hungary would draw in their allies as well, causing the conflict to become much larger than the two nations who started it. Further, industrialization meant that armies could mobilize quickly along carefully prepared timetables and that a country that got ahead of the others was at an advantage.
Moreover, the major powers that went to war in 1914 all believed that the only way to prevail was to use massive forces to achieve total victory. They were all willing to fight until the last man, even when it was no longer in their own self-interests and they had already squandered so many lives.