Life is wild in the Holy Stellar Empire.
The war has been going on for ten million years, long enough that nobody but the God-Emperor has lived to see its beginning. Nobody cares about history - with machines doing every possible job, all that's left for us humans is to party, party and die in the God-Emperor's sacred war.
There's laughter as a dance involving several men and women spontaneously degenerates into a group sex session. Nobody cares about contraception anymore, especially not the women - the only way to avoid drafting is to be pregnant or have a child under three, and modern medicine is advanced enough to heal any possible disease. The God-Emperor wants to have plenty of subjects, all to die in his ancient war.
Naturally the machines could just as well carry out the fighting, and they do - every major victory, every single step of progress that is ever made is accomplished by robots and artifical intelligences. But the God-Emperor is bitter, vengeful at a humanity that allowed his beloved to die in an accident. So to avenge the wife who died while checking on a supposedly harmless border dispute with the aliens, trillions upon trillions of humans and aliens both are to die.
The Holy Stellar Empire spans over a thousand billion star systems, with new ones colonized every day. Every day, millions of battleships are produced all across the Empire, and every day tens of thousands of fleets engage in battle with their alien adversaries. Every day, hundreds of planets, both those of the Holy Stellar Empire and those of the aliens, are wiped out of existence by immensly powerful planet-killers.
In the beginning the aliens tried apologizing for what had happened, tried placating the God-Emperor by gifting him countless of star systems. They did everything imaginable to make sure nothing would threaten the harmony that had existed between the empires for eighty thousand years. But the God-Emperor, who had only recently ascended to godhood with the help of technology, would not stop mourning for his beloved, would not stop blaming incompetent border guards on both sides. All those pleas went to naught, and so the fleets encountered each other in battle for the first time.
There is no true reason to this war, no real purpose - step by step, the Holy Stellar Empire is winning, but at a pace slow enough for it to take another ten million years for the aliens to finally succumb. For the last three million years, research AI's and think tanks of superintelligent enhanced humans have been coming up dry - there is no new technology to be discovered, no more mysteries of the universe that we would not have solved. Nothing can stop the steady carnage of the Imperial Jihad, nothing but a peace settlement which everybody knows will never come.
And every morning, as the day dawns on the ancient human capital of Terra, the God-Emperor stands by a window and stares into the rising sun. He stands in the palace of the holy capital, long since purged of any lesser beings, the soil sterilized for his undying glory. And there, staring right into the blazing star he swears that today, today my love, you shall be avenged once more.