In the grim darkness of the far future, there is only war. Awesome, awesome war. By the 41st millennium mankind has already achieved the pinnacles of glorious technology and then forgotten every single bit which didn’t explode and kill people. It’s a universe which replaces the laws of physics with metal album covers and then rocks even harder.
The golden age of humanity comes when Earth finally unites under a wise and benevolent leader to exterminate every other species in the galaxy. And we still screw it up. We lose our leader, piss off every other living and unloving thing in existence, and then start killing each other like we’re trying to finish the job before the aliens can start slaughtering us. It’s a race we don’t win, and that was ten thousand years ago, and it’s been getting worse every single day ever since. In this universe the second law of thermodynamics is entirely enforced by hitting things, and we built our own Satan to kill the good guys.
The only reasons humanity still exists is that our enemies are often too busy killing each other and themselves. Humanity tears itself apart as the Imperium crushes rebels, both are consumed by demons, all three are attacked by mech-suited space-communists, all four are assaulted by rampaging Orcs, all five are culled by hyperevloved star-elves, who join us in being tortured past death by hyperevolved evil star-elves, everyone is being eaten by space locusts from beyond the galaxy, and even they’re only alive until the immortal regenerating star-killing robot armies buried in half the planets in the galaxy finish rebooting.
The result is a crucible of war where badassery beats tactics, and humanity has some of the coolest combat equipment in existence.