The Imperium marches on.

The Imperium marches on, collapsing.

It began collapsing since the end of the Great Crusade. In that time - 10 thousand years, give or take - it lost over a half of it's worlds, now controlling around a million populated planets, depending on how you count.

For the entirety of it's existence, it have been constantly besieged from all sides.

A single Tyranid or Ork spore landing unnoticed on a planet containing organic material can doom an entire solar system. A single Necron tomb-world can reap a dozen. Pirates and slavers and warlords (and Dark Eldar, who are all three and in addition to that are pain-maximizers) raid planets left and right. Worlds regularly tire of endless oppression and secede (which dooms them to be brutalized - if not by Imperium's retaliation than by whoever comes there first instead).

But the worst danger is from Chaos. It haven't used to be quite that bad, but the birth of Slaanesh have spurred it's power, and the defection of Space Marine legions and with them entire planetary populations have provided it with fervent power sources and agents; subsequent misery and fear of Imperium feed it as well.

Chaos in the 41st Millenium is horrifically dangerous. A single psyker making a single wrong mental motion can become posessed, or worse, open a Warp rift; a single Chaos cult with a small human sacrifice or a moderately large peaceful ritual can summon a demon or forge a Chaotic artifact. Any of those things, unless they are stopped immediately, will lead to Chaotic taint, which will eventually lead to more demons being summoned, which will, in most cases, doom the planet to fall to Chaos. Resistance to a Chaos infestation at this stage will result in downright war, in which the side of Chaos is empowered by it and it usually has the option of summoning Greater Daemons, every single one of which can conquer a planet solo unless presented with truly extraordinary resistance, beyond the capacity of most planets to organize alone.

Which means that successful action against Chaos has to be done mostly preventively.

Every single citizen of the Imperium has to be indoctinated into the Imperial Cult and kept away from dangerous knowledge. Censorship and persecution employed by the Ecclesiarchy must be not just totalitarian but total. A full Chaos cult is already almost too late to stop - each individual Chaos heretic must be killed immediately before they recruit a cult. Except it really is hard to kill people fast enough to prevent people from spreading their opinions in private, so in fact, the heretic must be stopped before he even becomes a heretic. His education cannot include even pointers to pointers to Chaos. His own family must disown or better yet kill him for having any doubts. And as Chaos can be arrived at independently, it's Gods being derived from all-too-human impulses, anything that looks even remotely like Chaos heresy could potentially lead to the real deal.

The extremes of fanatical indoctrination, censorship and persecution are the barrier between Imperium and Chaos. A better civilization would find better ways to contain the threat. Imperium cannot see any other solution, and is only left with a feeling of grim necessity.

Moreover, every single Psyker in the Imperium has to be hunted down and given out to the Black Ships. Yes, Imperium is very bad at systematization and logistics, to the point of losing track of entire planets. But that is in fact largely a consequence of being forced to achieve near-perfection in such narrow areas as witch-hunting.

Of all the major factions of The Galaxy, only Humans and Eldar don't possess an extraordinary resistance to Chaos. And both Human and Eldar mechanisms of containment are extremely costly. This is not a coincidence. Most races either have such a resistance, or develop in areas where the boundary of Warp is especially thick, or are supremely paranoid about witches even before they know of their real threat, or develop under the aegis of someone more knowledgable... or fall.

Despite all the efforts, it's not enough.

There are, broadly, two main ways for a country to collapse.

The collapse can be logarithmic or exponential.

In a logarithmic collapse, the problems are proportional to the resources of the country. In such a collapse, the more the country's problems force the country to burn it's resource-generating ability, the slower they do so. A country collapsing in this way suffers a lot at first but then gently and slowly withers - potentially for eons, but usually until it gets too small compared to aggressive neighbours and slips into exponential collapse.

In an exponential collapse, the problems are more or less constant. To deal with them, the country must burn a part of it's resource-generating ability, which leads to it being less capable to deal with the same amount of problems the next time, which means it has to burn even more of it's resource-generating ability.

Between the two lies linear collapse, a fickle balance that may be maintained by a country expanding it's resource-generating ability to match the problems.

And beyond exponential collapse lies superexponential collapse, where the problems get worse the more you suffered already. When superexponential collapse becomes noticeable, you are probably already doomed.

Throughout it's long history, the Imperium have went though all four of these stages of grief.

The Horus Heresy and it's immediate aftermath, the beginning of the dark times, were a rapid logarithmic collapse. Though lightning-fast at first, as awareness spread, the number of easily corruptable people diminished rapidly, and by the time half of the Primarchs were corrupted, the other half were incorruptable. The Scouring of the Imperium and subsequent expansion efforts (which Imperium, by the way, continues to this day) have recouped most of the territorial losses.

But after the dust have settled, the balance of power turned out to have been shifted dramatically. Imperium have lost it's best leader, who was also it's best combat unit and best researcher. It lost a lot of it's territories, armed forces, and productive forces. It lost it's integrity and momentum. Meanwhile, the Chaos Gods have acquired a massive worship-power source and army, ready to do their bidding in the material world. And it started to slowly chip away at the weakened Imperium.

Imperium had to do something about it to prevent it's losses from actually weakening it. It's leadership was too weak and incompetent and plagued by infighting to muster all the human might at once and deal with the remaining problems once and for all. Instead, Imperium began to gradually trade off.

From the beginning, it was a militaristic state. But initially, it's militarism was akin to USA or Soviet Union of old in it's scale. Gradually, however, it turned comparable with Nazi Germany, then with North Korea, and then eventually settled at the level we know as Imperium proper. Modern Imperium funnels 100% of it's output into the war effort and counts absolutely everything else as strictly instrumental to that particular bottom line. Yes, a few of it's million worlds are inevitably Paradie Worlds - planet-sized hotels. But even the most militarized city with a million people in it would have a couple professional entertainers. The number of Paradise Worlds within the comparatively tiny Tau Empire far exceeds Imperium's, to say nothing of the Eldar Empire and Human Federation in the Dark Age of Technology. 

In addition, Imperium traded off it's freedom. From the beginning, it had an official ideology, a strict hierarchical structure, and the necessity of the Black Ships. It went through periods of thaw, all of which ended in disasters, followed by periods of oppression, which were unsustainable, but it was forced to gradually shift towards more and more oppression overall. 

Moreover, Imperium traded off it's reason. The Imperium already started with it's technical intelligentsia being a cult, focused on recovery rather than discovery and bolstered by a mysterious supernatural presence in the caverns of Mars; and with it's decisionmaking being centralized among a small self-selecting elite which no one was given a right to openly challenge. And fanatical conviction is a strong advantage in the world of Warp, not only decreasing the risk of Chaos and improving motivation and social cohesion, but also directly making the contents of convictions slightly more real. The contents of beliefs may have done a 180 degree turn, but their foundations simply progressively grew more and more pernicious.

For a long time, through such tradeoffs, the collapse of the Imperium have stayed linear.

Until the point where the Imperium ran out of things to productibely trade off. It was an utterly totalitarian war machine piloted by manic fanatics. Any more, and it would catch fire.

At which point, it finally stopped trading off.

Nah, just kidding.

At which point, it got a wonderful leader named Goge Vandire who doubled down on all three of those things.

And the Imperium, indeed, have caught fire.

Hey guys, what if we purged all gingers from this solar system?

And in this solar system, we forbid people to look at the sky. And install surveallence servo-skulls everywhere to enforce it. And have 5% of the people living there watch the camera feed.

Hmm. I edict thusly: All female children of Terra below 12 years old are now to be considered my property.

You know, if a guy with that kind of temperament came to absolute power immediately after the Heresy, when Imperium had slack to spare and was in dire need of suicidally decisive action, while the traitor forces were only just getting used to having the wonderful powers of chaos...

Maybe Imperium would have drowned the Eye of Terror in oceans of bodies and pain-earned fleets, and that would have solved all the problems.

But, uh. Uhhh.

Yeah this absolutely had to cease.

And so, the Imperium of Man have entered a new era of decay - exponential collapse.

This was signified by life actually getting better for the citizens in an aftermath of some reasonable reforms. The quality of life in the Imperium have surged out of the horror basement level and got back to what one might call the absolute ground floor.

The quality of life would continue to decay, but not as results of forceful increase of austerity but in the beat with the natural cultural shift, and as a result of objective factors such as decline of technological sophistication.

The victories of Imperium stop being earned by pain, and start to become earned by sacrifice.

Sacrifice of resources for winning the victories of tomorrow.

Which results in necessity of even bigger sacrifices tomorrow...

When the exponential decline have reached noticible levels, and the Imperium started to have to cede a lot of physical ground, the other forces started to reclaim it. Chaos got billions of new worshippers, Tyranids and Orks billions of tons of easy biomass, and Tau have established themselves as a major faction. In addition, Necrons started to wake up en masse - more and more and more...

Under superexponential decline, Imperium would be lucky to exist for 300 more years.

The prospect of somehow holding on for 2000 years? That would sound ridiculous to anyone brave enough to research the queation.

The entirety of Warhammer 40000 is a game, in which Imperium is one of the factions. And it has it's advantages.

A massive size. A nearly all-encompassing reach. Great wealth in material resources. Better centralization, cohesion and coordination than most other factions, even more plagued by infighting and disunity. A legacy of ancient technology and prosperity. And, of course, sheer fucking grit and fervor of it's people.

Other major factions have their advantages too. Some have great skill at precognition. Some possess an amazing power of not being intolerant militaint racial supremacists. Some can replicate very, very fast. Some possess numbers, material resources, reach, unity, technology, or even resolve much higher than those of Imperium.

But most of the resources and numbers and skills and mindsets that you start with don't really contribute to the true decisive factor, in the long run.

Because what truly decides the game-

-is the clock.

It is said: The hours of folly are measured by the clock, but of wisdom, no clock can measure.

By their very nature, a nature that remains constant across all worlds that you could conjure with an illusion spell, games are systems which state is influenced by the decisions of players in their effort to fulfill their victory condition.

In a slightly more high-level approximation, an approximation not of Game Theory but of Strategy that remains constant only across most imaginable worlds and most imaginable games, players make decisions to use options of committing their resources to trade them off in ways that increase expected value, seeking to expand their future range of options and contract the future ranges of options of their opponents.

That is to say, the connection between the player and the game lies through the decisions of the player. That is what allows the player to meaningfully pursue their victory in the first place. That is the difference between a game and an inanimate physical system.

Each and every choice made by a player with positive - however low - amounts of information and forethought and goodwill has a positive impact on their expected value. Some decisions will certainly end up impacting it negatively, but quick subsequent decisions may often undo or compensate for them. And getting better value allows for more tradeoff options later, so that each decision gives you even more value. And so on.

Good quality of decisions helps a lot, for sure, but "above zero" is, in principle, enough. With enough edge in frequency, quantity becomes quality, and in fact overwhelms it.

And that is why the-frequency-of-meaninful-informed-forethought-goal-directed-decisions is the king.

That's not to say that other advantages are nonfactors, of course, but they are not nearly as decisive.

Those with a faster clock win in the long run. They have more actions to push them to victory. Not just every diplomatic or military action taken, but every law enacted and dictate proclaimed, every novel research paper finished, every major business founded and every major investment made. And the more things move into the spave of what can even be considered with reason, the more things are added to the list - for a species practicing eugenics, every new trait chosen for encouragement or discouragement is a decision as well...

The Tau have made their clock fast indeed.

But those who abandon adaptation for tradition, those who sacrifice sober judgement for faith, those who only make decisions if they are forced into them by circumstances outside their minds,

break their clock in half.

For all it's power, for all it's reach and importance and for all the billions of sharp minds behind it, there's a sense in which Imperium of Man isn't a faction, but a piece of scenery. 

...

Earth is ancient. For a third of the Universe's existence, it stood firm.

When the Galaxy was aflame with war, it haven't been entirely unused or untouched. But it survived the devastation without much overt change.

Man was created, and rose from the dust-fringed edges of the cooling mud. To love, to conquer, to kill - AND HE WILL.

When a race with a mind structure of significant Warp sensetivity reaches a level of sentience and sophistication high enough to develop proper psykers, the race encounters a deadly test. Because if a psyker was to stare for too long at the Abyss, the Abyss will stare back. A single minor daemon summoned and not stopped in time can and will corrupt people into the service of Chaos, who will then summon more daemons through rituals no longer requiring a psyker; the process will repeat until the new race falls, and if it halts partway by inordinate amounts of resistance, the already empowered corrupted faction has the option of summoning a Greater Daemon, each far more dangerous than most monoplanetary civilizations to ever exist.

In the ancient times far before Slaanesh or indeed before Tzeentch, summoning demons fas far less of a trivial task, as they were both less numerous and less powerful. But worlds can fall to more subtle influences of Chaos than that. Cults devoted to entirely unrelated things twist over time into the worship of the Ruinous Powers. Metaphors of inspired poetry and whispers of dreams slowly, slowly bend cultures towards ruin. With time, water cuts the stone.

Some worlds are lucky enough to contain Warp-suppressing Necron pylons or other artifacts of similiar function. Other lucky worlds are located in areas of space where the boundary between Warp and realspace is unusually thick. Some young species have been forewarned by their more knowledgable neighbours, often while being conquered by them.

But the unlucky worlds with normal conditions usually only have a shot at long-term independence given strong traditions of both witch-hunting and cultural stagnation, which have to begin and become universal before the true dangers of witchcraft and cultural drift become apparent.

But there is time before one of such unfortunate changes happen.

That is the time in which free users of the powers of Warp may walk peacefully among their people and practice their craft.