The Great War in the Bell, Book and Pistol universe (due to the lack of the Bolshevik Revolution and the lack of Communism and Socialism) took the fate-threads of our two world wars, and merged them into one, due to the differing development of history, society, culture, and government. The Great War was started by the assassination of Archduke Franz Ferdinand, heir to the Austrian throne, by a Serbian. Keep in mind that by this time in history, the map of Europe looks a lot more like it does today, because certain central european countries refused to give up their monarchies, and had the magical support of the land to retain them. The Ottoman Empire could not conquer Romania (and Romania retained Moldova), thus the Empire at its height had only reached Turkey, Georgia, Bulgaria, Serbia, and Macedonia. At the time of the assassination of the Archduke, the Ottoman Empire was down to just Turkey, Georgia, and Bulgaria. The United Kingdom, France, Spain, Greece, Russia, Ukraine, Belarus, and Poland sent troops to stop the invasion of Serbia. On the other side, Germany, Austria, Slovakia, Hungary, Bosnia, and the remains of the Ottoman Empire stepped in to invade Serbia. Later in the war (which lasted from 1914 until 1945), Italy joined the Central European Confederation, as did Japan. When Japan entered the Confederation, China immediately joined the Alliance. India, which had been united in the Satavahana Empire for almost 1900 years, had thrown off British rule like a bad habit, and joined the Confederation.
Since Italy had joined the Confederation, Pope Benedict XV called all Catholic nations to join the Confederation. Thus most of South and Central America, Ireland, and Mexico joined the Confederation. Canada, Australia, and other Commonwealth nations joined the Alliance. Eventually the countries of the middle east joined the Confederation. Siam joined the Alliance.
Now the major difference in this Great War was that although it was fought with troops and tanks and planes, it was also fought with shifters and mages, and any vampires that could be bribed to take part. Thus the Great War is also known as the Great Mage War. There were mage wars going on in central Europe, Africa, Asia, the middle east, all over the place. However, the mage wars on the US / Mexico border were bloody as anything in Central Europe. None of the Allies magical attacks could get through Japan's defenses, however, because they performed special magic that used the lives of its Kamikaze pilots to strengthen Japan's magical defense shield. It was due to a series of German defections (which had nothing to do with anti-semitism in this universe, because Turkey and all Christian nations had a healthy respect for their spiritual ancestors in Judaism) that the US gained the technology and understanding of Physics to develop the Atom Bomb. The atomic blasts on Hiroshima and Nagasaki happened at the exact same time and manner as in our world. Japan surrendered immediately. However, the war elsewhere still raged on. Switzerland, Romania, and the Czech Republic, who were neutral in the war, but sustained severe damage anyway, secretly contacted the Elves, asking for an intervention in 1932. In 1945, the Elves got pissed off enough that they invaded through their Gates from Elfhame, and forced a surrender of all parties in this war. What can I say, time flows differently in Elfhame, thus the long delay in the arrival of the Elves was mainly due to the fact that 1 year in the EarthPlane is about a month in Elfhame. The Elves seized Ireland, and all woodland areas surrounding their Elfhame gates, taking away most of the wilderness areas from humanity. When the humans resisted, the Elves let them know in no uncertain terms that if the wild areas were not returned to them, that the Elves would wipe out humanity. There are still humans living in Ireland, but under Elven rule.