"Welcome to Warhammer 40k!" Be the Imperium of Humanity in Warhammer 40k. You receive word of a Chaos incursion upon an Imperial world. Immediately declare a Crusade, send in everything you can: Imperial Guard, Space Marines, artillery, vehicles, even a titan or two. After 80 years, which might be considered a surprisingly short period of time for such an occasion, you finally do it, you manage to expulse the evil forces of Chaos from the planet. You've taken enormous casualties, the civilian population is basically extinct, and the Death Korps of Krieg have officially declared that this is their new favorite place, but the minions of The Enemy have been denied their prize. Then the ground beneath your feet shudders unnaturally. The planet is a Necron Tomb-World. Your Crusade has roused them. Now you have to confront the Necrons. At the same time, a space hulk comes crashing down through the atmosphere to impact upon what used to be a very important manufactorum. It's an Ork Rok. A WAAAGH!! comes pouring out of it. Now you're facing the Necrons AND an Ork WAAAGH!! In the end the Space Marines are called away because the Chaos warband you just drove off-world? It's just struck a neighboring planet, and THAT one's a Forge World, so the Adeptus Mechanicus are DEMANDING that the Astartes be present. The last titan you had got taken out by a pair of Ork gargants. The Necrons have relentlessly slaughtered the Imperial Guard to almost a fraction of their former numbers. Orks are overrunning your positions. After some brief deliberation, it is decided that there is no real reason or purpose for this world to exist anymore, as it is no longer useful to the Imperium. The Order to evacuate the planet is given. Few actually receive this order. They're too busy fighting for their lives. Exterminatus is declared. The planet and everything on it burns. Humans, Orks, Necrons, everything is utterly annihilated utterly. The world is now a desolate dead rock in space. You declare it a victory over the enemies of humanity and go on to see how the Space Marines are doing liberating that Forge World. WELCOME TO WARHAMMER 40K.

Don't forget that the Imperial Administorum has misplaced the war material supplies that were supposed to go to your world, due to dropping a sheet of paper that slipped under the desk. They got routed to someplace on the other side of the galaxy, and no one's going to notice for another 1500 years. Otherwise, perfect. Everything checks out.

@squidlord @Ochadoji999 You forgot that the same Adminsitorium is still requiring your Tithe and that your extention period is past so you owe 40 years of back tithes

Absolutely. Though they may forget to come by and pick it up for a hundred years, even after you acknowledge that you owe it. When they do, they'll bring a handful of genetically uplifted demigods and a legion of guys with t-shirts and flashlights who don't know who you are and don't care. Not a single one of them will help out because they'll arrive 25 years after the planet has been turned into a blasted desolation. They'll still blow up a couple of mounds of rocks just to say they were there. Just after they leave, the ork spores will begin sprouting.