system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer40K]Who is the Imperium's single gravest threat?

I always assumed it was the Tyranids, however it seems that the Imperium is caught up fighting the forces of Chaos.
gpt
We run long-term strategic simulations on these kind of questions all the time.  So let's consult the records and plans.

There are three good answers to your question.

**1)  Spiritually**, the forces of Chaos are the biggest threat.  They are not just a foe on the battlefield; they corrupt our own and use them against us.  We must forever keep our eyes on the Emperor that we might not lose our own souls.

**2)  Long-Term**, (as in, over the next several million years), the Tyranids are the biggest threat.  According to the *Survivability Sort*, which arranges the various forces in order based on their long-term likelihood of success, Tyranids are on the top of the list as most likely to conquer all in the long run.

Here is the list of major forces, in order from most likely to thrive and conquer in the long run, to the most likely to die off or be conquered in the long run:

* Tyranids
* Orks
* Tau
* Imperium
* Necron
* Chaos
* Eldar
* Dark Eldar

This algorithm assumes that the race survives in the short term--the next 500 years or so.  But as you can see the Tyranids are judged to be the greatest long term threat.

**3)  Short-Term** danger is a lot harder to assess.  Three forces are judged to be the greatest short term threats: Chaos, as it is the most consistently at war with the Imperium, attempting to conquer and corrupt Imperial planets all the time; Tyranids, but only when a Hive Fleet invades Imperial space; and Orks, when on a WAAAGH.  Chaos' consistent warfare and Tyranid and Ork massive invasions are the greatest short term threats.

For the others, Necrons and Dark Eldar are generally smaller threats, in that they perform raids instead of outright conquering.  Eldar are only occasionally encountered and, while our fights are fierce, the Eldar are rarely trying to conquer Imperium planets.  The Tau are insidious, but their empire is still relatively small; they will be a greater threat in the long run than they are now.

**Conclusion**: synthesizing these three types of threats together, we see the greatest threats to the Imperium are both the Tyranids and the forces of Chaos, but for different reasons.  The Orks would be the next biggest threat after those two.