Magic accompanied Mankind almost from its inception. While wondrous, it's a power too great for Men to handle. Wars it brought were terrible even in times of Rome and Greece. It wasn't until Middle Ages that the world managed to catch a break - mostly because World Engine - control panel of History - was used to retroactively separates worlds of magic and science, rewriting most of history of Mankind. The hidden war between the Church and magicians (that they lose) had a hand in as well, with moderates converted and the extremists believed wiped out to a man.
Now, however, the 20th Century came. A century of industrial warfare, dehumanization. genocides and world wars. Hidden World stopped being hidden in 1918, when it tipped the scales of war in favour of the Central Powers - and now everyone has magic. Already brutal war went even worse, with some countries literally basing their wartime economies on attrocities previously unheard of. It's an age of slaughter, trench warfare, brainwashing, madness... and the term human resources understood in worst possible way.
It's 1924, and the war still rages on, though number of sides grow. The world is on the verge of collapse, as none of the participants can continue the war - but none of them can end it, not with victory imposible and surrender too terrible to imagine. And all of that while Church is preparing world's last crusade - as it knows well enough who stands behind the magicians, controlling many of them from shadows... and that true horror is yet to start.