Overall series information: this series will not be nice to the things referenced, any realistic fiction elements are just that, fiction. Only the designs and "types" are taken from Pokémon, nothing else. War is also a part of every chapter as it is part of the setting.
One day a sylveon looked up at the machine he made, his life work finally completed and ready to test. The sylveon goes to check his system, he had hacked into the entire world's entire short range teleportation complex, he was about to do something that could make them obsolete but needed their power for his first test. He then plugged in his machine to the generator he made specifically for this purpose. Filled with excitement the sylveon ran up to his controls inside the safety pod he built. Locking himself in he presses the start button ready to watch his efforts pay off. He was ready to create a rangeless portal.
The machine starts up pulling all the power it has access to, which ends up being more than it could handle and the machine begins to overload, in his haste the sylveon forgot to put an end point for his portal. The overloaded machine begins to create an orb that absorbs anything near it, after a minute, the orb seems to collapse as the entire short range teleportation system overloads at once.
The sylveon wonders what he did wrong. But his thinking was interrupted by the machine, it's not actually done. From where the orb once was, light extends and spreads out forever. The sylveon looks out the window to try to see what happened he sees a rift, at first he thought the rifts were just consuming the world, until something comes out of the rift, a bipedal creature with hair just on its head. "Oh no" he thinks, "best case scenario I just uncontrollably linked worlds and worst I have uncontrollably linked realities." It was the second.
The rifts around the worlds lead to chaos all over both worlds, things appearing and disappearing, criminals using the rifts to get far away from the authorities, creatures going missing and showing up randomly. It was inevitable that the governments contained in both worlds would interact, and it didn't go well. Both blamed each other for the chaos, they didn't see what was going on in the other world, they just saw that the chaos and the creatures from the other world appeared at the same time. The relations broke down and the human's United Nations and the eeveelution's Intergovernmental League declared war on each other.
The rifts allowed both sides to take out each other's weapons of mass destruction before either could get any control over the rifts. But even though weapons of mass destruction were removed from the war this didn't mean the war wasn't going to be very destructive to both sides. Over time the rifts were put under a little control by both sides, they couldn't create any rifts but they could restrict where they could form, but it was so difficult that only the most important places were covered. The rifts have to be worked around for both sides. The war is intense and no end is in sight.