Nobody could remember how the war began, and by now nobody could imagine it ending. Nobody could make peace with the Vikings, and their continual sneak attacks meant nobody wanted to. Our three mega-nations are locked in this constant state of nuclear war for good.
They say hundreds of years ago there used to be ice at the poles of the planet. I would have liked to have seen that. All we have now are warm seas and swamp everywhere, most of it irradiated and unusable. The people in every nation are starving, but we all work non-stop to build the nukes that keep the war going. It's fight and live or eat and die. There are no pacifists any more.
It took years to get going, but we Celts and the Americans formed a fragile, uneasy and secret alliance against our only other enemy, the Vikings. It was difficult. It was risky. More than once the talks erupted into fresh nuclear strikes and everything went to hell and back again and again. But for a few short months, we managed to become allies.
Neither side trusted the other to attack the Vikings without sending a few missiles their way, too. The joint assault force, if it could be called that, was a single submarine crew from both nations, sent to take out the Viking capital city, currently believed to be in the north. They called it Asgard, of course.
Tensions were high in that submarine as elite soldiers from both sides held back grudges begun by their great-great-great-grandfathers. They worked with their weapons in their hands the entire time, casting dubious glances at one another around bulkheads and across the small dining area. They slept on separate decks, with the doors locked. There were occasional fistfights. But in the end, the submarine and its unstable human cargo arrived at Asgard.
That was when the Celts made their move. As the hatch slowly opened, the Americans exited first, but the Celt commander pulled the hatch shut from the inside, stranding the Americans outside. As the Celts submerged again to make their way home, stranding their would-be allies, the American shouts of protest drew the attention of the Vikings, already waiting in what was supposed to be an abandoned dock.
The battle was brief and one-sided, and, as it ever has, the war rages on.
Mokalus of Borg
PS - Inspired by this Reddit post about an eternal war in a game of Civilisation II.
PPS - Others have written their own fiction in this world, too.