World War I, known as the Great War or erroneously and infamously as the 'war to end all wars' was the first globalized conflict on the planet Earth where all major powers and a vast majority of the planet's nations were embroiled in conflict.
There was no true singe cause of World War I, no matter how much the eventual victors attempted to pretend otherwise. It was an outgrowth of centuries of hatred and distrust, combined with an absolute mess of intertwining alliances and mutual defense pacts. All it took was a single match to ignite the fire, and that match took the form of a botched assassination of an Austro-Hungarian archduke.
Aside from the truly ghastly nature of a global war, where hundreds of thousands died in trenches, what makes "The Great War" so notable is that it was the first time that Type-II individuals openly fought as part of national militaries. Just as technology advanced by leaps and bounds during the war, so did the world's understanding of RM and its properties. This knowledge was not without cost, however, as thousands of mages would lose their lives in the process, cut down by either their counterparts, massed gunfire, or choking gas that not even they could long withstand.
The early days of the war, before anyone truly understood how devastating both Areum and the results of industrialization could be, were viewed something wonderful, glorious, and above all, done by Christmas. As is in the case of all wars it seems, the generals had not learned the lessons of the previous Revolutionary war, and so forth the armies marched, resplendent in colorful clothing which made them nearly impossible to miss on open ground. And miss their enemies did not, as machinegun fire proved to be even more devastating than sorcery against soldiers thus caught out in the open. These technological and magical advances resulted in the western front grinding to a halt, as both sides dug in and found themselves unable to dislodge their opponent.
The Eastern front was more mobile, partially due to the land it was fought on, and partly due to military blunders on both sides that allowed the opponent to advance. Austria Hungary proved incapable of subduing a nation many times smaller than itself on its own, and Russia had its own difficulties with untrained troops, fewer volunteer mages, and lack of equipment. And even despite the staggering losses that all sides of the conflict were suffering, each year brought new nations into the war, tempted by promises of territorial gains when the war was won.
Though the war would be eventually won, it would not be without cost. Multiple countries, the UK and Russia among them, instituted a universal draft of all Type-II individuals, many of whom would never return home due to being treated as super-weapons and thrown into the worst of the fighting. Though Type-IIs bore a disproportionate amount of casualties, that is not to say that normal soldiers were spared. Few in power were concerned about the fate of the common soldier, Gallipoli is proof enough of that. No country that fought was exempt from casualties, though some were better off than others. At the end of the war, none were unscathed, none were unharmed.
The immediate aftermath of the war brought the question of what to do with the innumerable soldiers, wounded either (or, worse, both) in the body or in the mind. While the victorious powers, France and Britian mainly, could simply ship off these unpleasant reminders of war to their colonies, Germany could not, and the sight of a crippled, uniformed soldier begging on the street became horrifically common. Another result of this in Germany was that ex-soldiers, unable to find work due to the devastation wrought and the horrifically punitive Versailles treaty, began to join militias, basically unofficial armies, under the command of the political parties. Including a small, marginalized party known as the "Nationalsozialistische Deutsche Arbeiterpartei" or the NSDAP for short.
The legacy of the Great War was one of horror and suffering. Due to losses in battle, and the feeling of betrayal at being drafted and treated like living weapons, the Type-II population of Great Britian and France went into hiding. Those in Germany and Russia turned their anger to "more productful" outlets, like throwing their lot in with certain fringe political parties. But the most important part of this legacy stemmed not from the war itself, but from the treaty that ended it. Versailles, the hated treaty, and one of, if not the, main cause of World War II.