Originally Posted by
Arbitrarity
Okay, so assuming the "Great Leader" is some variety of Titan, I'm not sure I follow their reasoning. Usually I hear Titans want revenge on the gods, or weird metaphysical stuff to do with their aspect (cover the entire planet in ice, and so on). The former seems like it shouldn't necessarily apply in the Fate universe, because none of that Tartarus-sealing stuff happened, and neither seems like a very effective way to convince ethical godlings to join your side, as far as inspiration goes. I guess you only need one bad apple with Epic Appearance/Manipulation/Charisma, then you can get bonuses for familial loyalty. Assuming you can turn one of those with an appropriately capable Titanspawn, you're in good shape. You can then make convincing arguments that Erik's paranoia is blinding him to negotiation and so on, that the entire Hunter's Dream thing is a moral nightmare, and so on.
But the entire "we've been attacking humanity for millennia with Frostspawn" just doesn't make sense if you have a reasonable negotiable position, unless Erik is completely impossible to reason with and you need to prevent him from building up enough forces to actually remove your side. Which isn't out of the question (actually seems probable), but that definitely means he isn't necessarily in the right.
It sounds like the pitch for godlings is "let's make a better world, especially for gods, this may involve breaking some eggs" or something like that, which definitely appeals to the undoubted superiority complex of many gods, regardless of parenting, and also to the sense of "the big picture". Given enough detail, this is sufficiently convincing. But it seems like they're committing atrocities either for convenience, or because Erik is impossible to negotiate with, unless there's some metaphysical property of having many humans around that is problematic... which is Alaya in the Fate universe, I suppose. Ohhhhh. Yes, I can see why that could be a problem.
Right, so their leader is probably some aspect of Gaia (Archetype Earth, for example), and for planet-preserving purposes, humanity needs to go. That still seems like a compromise could be negotiated if you could get Erik to evacuate everyone off-planet, but that would take a long time (also the logistics would be hell, given I hear there are literally millions of humans still?). Still, they've had thousands of years that they've spent on conflict instead. Also, you probably can't trust Erik to not come sailing back in and blow the place to hell the instant he thinks he can reclaim it. That last one is probably bad enough that you have to subdue him, which then kinda justifies the rest. That... still kinda makes him the problem.
Of course, there's also the possibility that the enemy doesn't care about possible compromises, expects to win, doesn't value the goals of Erik/Shirou in the slightest, and has decided that the concessions of such negotiation (and extremely great risks of later reneging by Erik/Shirou) are greater than the effort required for total victory. That does match the expected profile for Titans and so on, and given the above arguments, allows them enough wiggle room for being convincing to godlings. After that, they can use similarly convincing arguments to avoid tipping their hand to Erik.
Personally, now that some sort of dialogue has been opened, I'd like to see some effort by Shirou to ask about those points, why evacuating humanity is/was not an option, why seemingly no effort was made to negotiate, and so on. Erik estimates it can be done in 1000 years from 1500 AC, so that is still a long time, but he's kind of hindered by the constant conflict. Also, I notice the Enemy is running Holy Grail Wars. If Erik has any demands, and/or they need to geas him for contract, couldn't they use one of those to enforce it? Sure, the Grail isn't omnipotent, but enforcing a willingly accepted contract, even on a God, and speed-constructing a giant space fleet should be within its parameters.
Anyways, assuming that no, all humans except for like 20 must die, and so on, I think Shirou... would fight. He has a tendency to take responsibility for everything that comes about directly or indirectly as the result of his actions, and is very sunk-cost-fallacy. After spending millennia shepherding humanity, he's not throwing that away. Also, that's probably the form of his legend, unless it's been sufficiently manipulated by the other gods (noting the "to counteract your Fate that my siblings worked so hard to become fact for you"). On the other hand, having him flip would give a lot of currently obscured setting information that would be interesting.
EDIT: Also, sufficiently competent enemy infiltration definitely allows for messing with even the most well-planned religion, particularly over thousands of years. If Shirou hasn't been very carefully watching rumors (and he likely hasn't, given he's been withdrawn), it's quite possible Fate is going to coerce him in either direction based on what his people believe about him. Spreading enough "is lonely, has fated love with Semiramis" belief could have an effect (though I expect in Scion rules, unless those effects scale significantly as your legend increases, even incredibly strong fatebonds don't have nearly as much effect as high-level Epic Charisma and so on)