Who'd be in charge if...

[treadiculous]

In a conflict with Space Marines and Imperial Guard vs xenos; who'd be in charge?

What about if adeptus mechanicsu had involvment too?

Are there other positions beyond the inquisition?

what if the pace marines had a very small force, and guard the much larger?

[cheminhaler]

The Soul Drinkers series by Ben Counter has a fair bit of marine/ guard interaction and I feel he nails it better than most authors. Marines are basically just children who've never grown up because they're taken from a young age and taught that they are the best, and it breeds arrogance in them. The guard usually are in awe of them because of their abilities, rarity and the stories about Marines that are part of their legend and the heritage of the Imperium, so they're willing to follow the marines into the teeth of hell, even if it means disobeying direct orders from their own High Command structure to achieve the marines own objectives.
Sometimes marines will have differing objectives from the guard, e.g. Dark Angels have their own agenda etc. and it grates on guard commanders that their troops get used by other organisations but most of the time they have little say in who/ what gets commandeered. It is possible for desperate marine commanders to take guard out of the action even if they directly cause the failure of the guard commander, if it means achieving their (loftier in their mind) ideals. Doesn't matter about force size guard will always defer to marines because they are elites, ranks don't matter too much either.

Guard/ mechanicum tend to be equal allies but fight as their own armies, keeping themselves to themselves. When things get desperate mechanicum will ask the guard for their unused dead 'flesh units' (I.e. the corpses) so they can make basic servitors out of them. Dark Apostle by Anthony Reynolds features Elysian drop troops fighting with AdMech against the horror of the Word Bearers.

[RedsandRoyals]

"It depends", is your answer.

Depends on the enemies, depends on the scale of the campaign, depends on the experience and forces at the disposal of the commanders in question, depends on the objectives of each force. For the sake of convenience, we'll leave the Inquisition out of this, and come back to them further down.

Broadly speaking, if a Chapter Master is on the scene, and is inclined to take "command" of the local conventional forces, he's going to get it. See the Armaggedon wars, or Shield of Baal. This gets dicey if it's a chapter with a bad reputation (Flesh Tearers) or if the marines have different objectives than the local forces. In both cases, there are references to Guard or Navy commanders refusing to fight beside them, or 'declining" their aid. They're well within their rights to do this, obviously, but it does run the risk of the Space Marines packing up and going to find some other war, which may be less than ideal for the regular guardsmen who are up to their armpits in Hrud or whatever. As Chem noted, space marines can be big babies if they don't get their way.

With the Mechanicus, generally speaking they will support the other Imperial forces, or at least cooperate to whatever extent possible if their objectives aren't similar. The Mechanicus is pragmatic enough to realize it's better to let someone else do most of the fighting and dying, as long as they still get that tasty archeotech or secure the locations they're after. There will probably be some give and take in the relationship, though, with the Mechanicus demanding that only their troops deploy in a certain area, or a particular forge can't be bombarded, or, in the case of fighting the Tau, declining to send their Titans out to play if they think there's a good chance they'll get slagged.

That's on the strategic level, though. On the tactical level, cooperation would most likely be ad-hoc unless worked out ahead of time. If the Marines and Guard are cooperating, most likely the marine Sgt will leave command to the Guard officer unless he's blitheringly incompetent or all the officers are dead. I can't imagine most marines would want to bother taking the time to micromanage a guardsman platoon in addition to coordinating with their own Battle Brothers. It's not really practical or useful. They'd probably try to come to some sort of working agreement before a given assault about who's doing what, just to keep things from becoming too confusing. The same is probably true with the Skitarii, but it wouldn't be too hard to imagine them sort of just "ignoring" disordered guardsmen.

So, short answer? There will most likely be some consensus building on the strategic level, and possible at the tactical level, but for the most part, everyone will mind their own troops. I'd argue the Mechanicus would be the least likely to be put in charge, since that's technically giving Imperial assets to a non-Imperial organization, and the Mechanicus would probably be the least likely to go "Okay everyone I'm in charge here". The alternative is everyone does their own thing and risks stumbling over one another (or screwing one another. See the Taros campaign).

With the Inquisition, it REALLY depends on the Inquisitor. The Guard has to follow them if they take command, unless the highest ranking Guard officer can count on some level of protection due to connections or reputation. If the Marines or Mechanicus can establish a good working relationship with the Inquisitor, and see the advantage to coordinating with them, they'd probably march to the Inquisition's beat, or at least defer to them from time to time.

[that1guy]

Some chapter's are more likely to seize command than others, some chapter's are more likely to schmooze co-operation, and don't forget most chapters have their own PDF already at their service. It's different unit by unit, regiment vs. chapter

What you are asking is really up to the politics of the parties involved.

I just read a great book, Calagar's Siege, it was just as good as your average Graham McNeil Ultra Marines adventure. And the Chapter master of the space smurfs was well with in his rights to command all citizens with in his sector. Infact the planetary governor struggled with inner turmoil, unable to decide if he felt relief at the responsibilities of command being lifted from his shoulders or if he held apprehension at how nonchalantly, unceremoniously & quickly the Chapter Master relieved the mortal of command.

Keep in mind, this was in the Ultra Marines sector so they were with in their jurisdiction.

NOBODY in the Guard is going to command the Legion of the Damned ever, that is for sure.