How would nazis react to the arrival of aliens?

[ChudoJogurt]

I'm running a D&D 3.5 game, though the precise system doesn't matter in this instance.

In the setting, there is a fairly isolated region of the new continent, where the PCs are heading. To explore it.

This region, which is sort of vaguely Dark Suns-inspired desert badlands, has a bunch of factions, and one of them is here's an essentially fascistic organisation, living in a very isolated biome. They call themselves 'The Pure'.
The Pure have a monopoly on some forms of technology in the region (though not the most advanced one), and they all are goose-stepping militaristic boot-on-your-face shaved-head kill-the-heretic-burn-the-mutant types and are very focused on all sorts of things related to racial purity, hunting down people with magical mutations and imposing a strict militaristic hierarchy on places they control. They aren't _explicitly_ supremacist on a racial/subgroup/etc identity level, especially since their isolated place is all mono-racial, with no one but the predominant race having been there essentially forever, but they operate under the belief that their way is the only way to hold everything together and survive in their rather inhospitable environment, and that due to their discipline and order, they are the superior faction, and they do tend to call everyone who does not conform to their version of ordnung degenerate and [subhuman] barbarian filth.
So, fantasy nazis.

Now PCs are about to arrive there, and PCs are a rather diverse bunch of various D&D races. They come from a more advanced civilisation, so they have more advanced magic and equipment than anyone in the region, and they have their own system of hierarchies and aesthetics. They aren't however, any sort of mutant magical or otherwise, and they don't look like filthy barbarians. They're just different.

Obviously relationship between them and the PC and PCs home Empire would depend on PCs actions, but what I'm trying to figure out is what the initial reaction of a society that is built on the idea of superiority and conforming everything to its idea of hierarchy would be to the arrival of something that doesn't really fit into their idea of hierarchy, and strongly casts their superiority into doubt.

[HoboKnight]

First of all, Nazis were a cult. There is no re-evaluating the beliefs and standards, so pretty much nothing can shake it. Second, PC visitors seem to be powerful, connected to another powerful civilisation. Great! Treat them as honoured diplomatic guests, learn from them as much as possible and try to assimilate the technologies/powers they have. I think equivalent would be a Soviet diplomat in Berlin in 41' or an Ottoman diplomat in occupied Spain during islamistic occupation. You do not rage/attack/humiliate such visitors. You serve them best drinks and food and give them best lodgings. If one of them creates a mess, he is escorted to the border(politely) and paid a one-way ticket back home. Power transcends racist friction.

[Vahnavoi]

Short version: denial, anger, bargaining, depression, acceptance.

Long version:

Part of your premise does not make sense. This kind of hierarchical thinking requires a contrast point, it requires some level of diversity to take off. If the society is secluded, then this diversity must exist within the society: in practice, this "mono-racial" society must not view itself as mono-anything. If not race, then it is sex, caste, class or some other dividing line that explicitly makes the difference between the Better People who deserve to be top and the Inferior People who deserve to be on the bottom. Additionally, to keep in line with the idea of seclusion, in order for the society to have lasted in this form, the "inferior people" themselves buy into these notion. This is typically what modern players get wrong about this type of Lawful Evil society: they can't accept that the people under the boot would themselves believe they belong under the boot and be active part of their own oppression.

In any case, the exact vectors of oppression, the exact dividing line within the hierarchical Lawful Evil society influence how they react to total outsiders. For example, suppose the dividing line is sex, placing men on the top and women on the bottom. Well are these outsiders men? Or are they women? Depending on the answer, they might not sway opinions of the Lawful Evil folks at all. Confirmation bias goes a long way to make people focus on minor similarities that fit their narrative while neglecting more radical difference. On the other end: how different are these outsider? If they are really different, then a case can be made that how they do things just doesn't apply to the Lawful Evil folks. As long as the outsiders and their empire stay Over There, Far Away, they can be ignored for everyday purposes.

In order to cause major upheaval, then, the outsiders have to be similar enough to the Lawful Evil folks to cause self-reflection and different in ways that directly challenge Lawful Evil expectations. They also cannot be a passing fad, they need to be there to stay.

So, several outcomes can be imagined:

1) Disbelief: members of the Lawful Evil society just don't buy that the outsiders are who they say they are. You can expext ridicule and attempts to discredit them. Common folks might not believe they actually exist.

2) Hostility: this is the point where the existence of outsiders and their empire can no longer be denied, but their superiority can be. So the Lawful Evil folks will take their opportunity to show their way is the true way. This doesn't necessarily mean military aggression, though it is likely; it can mean cultural or religious aggression, starting with them trying to sway and convert individual outsiders (though it may extend to destroying them if they are perceived as members of the Inferior People).

3) Border Control and Protectionism: even if open warfare is not possible, maintaining strict separation between worlds might still be: outsiders are only allowed in special territories, most Lawful Evil folks are kept from interacting with them and those who are allowed to interact with them must pass some ideological purity test. On some level, you might start to see acceptance of the idea that things are done differently Over There, but that is not Here and does not and could not work here.

4) Social disarray, dissolution of order and civil war: this happens when the cultural pressure of the outside empire can no longer be kept in check by the Lawful Evil folks and ideas that directly challenge their cultural notions start to slip through. This is when some of the Inferior People might start to think they're not so inferior after all, creating grounds for a rebellion or revolution; on the flipside, you might get an even more radical forms of supremacism lobbying for more extreme measures against the outside influence. Importantly, however, much of the new aggression is turned inward: the new supremacist will be fighting the old supremacist and some of the Lawful Evil people will flee towards the outside empire, not because they've changed their beliefs, but because their beliefs are not sufficiently crazy anymore.

5) Historical revisionism: whether it's the old supremacist or the new supremacists who win, either way, sooner or later, someone will have an idea of integrating the existence of outsiders to their dogma. So they will come up with some rationalization why the existence of outsiders doesn't threaten their supremacist beliefs and then, importantly, start to pretend this is how they thought all along. If they get away with it, eventually all of the Lawful Evil people will believe they always knew of the outside world and their True Way always answered the challenges posed by it, no matter how dissonant this is with real history.

6) Point of No Return, cultural collapse and assimilation: this happens if resistance towards the outside world proves fruitless or if the outside world retaliates with sufficient force. The cultural beliefs of the Lawful Evil folks are made powerless and they are slowly wiped out either physically or culturally; their children, if any remain, will learn and speak the outsiders' language, wear their clothes and listen to their music.

7) Cultural Revisionism: this may happen relatively fast, during the first contact with the outsiders, or way after the fact, when no genuine members of the Lawful Evil people remain. In any case, the Lawful Evil people are reinvented as noble savages of sorts; superficial elements of their culture are put on a pedestal while elements that conflict with contemporary elements of the outside empire are minimized or selectively ignored. People will appear out of the woodwork, suddenly insisting the Lawful Evil folks are or were the Pure holders of the One True Way, despite the fact that these "pure holders of the one true way" physically and culturally lost to the outside empire.

[Maat Mons]

I’m pretty sure the two possible reactions an individual can have to something that challenges their worldview are to reconsider their beliefs or to become increasingly dogmatic. I’d expect some individuals to go one way and for some to go the other. I wouldn’t worry about devising plausible rationalizations for the people who double down on the old ways. Repeating something often enough an emphatically enough can compensate for a lack of supporting facts.

[LibraryOgre]

The Mod Ogre: Be aware that this thread treads VERY close to the "No Politics" line. Confine your answers to the fictional world created, and avoid examples from history.

[Psyren]

Now PCs are about to arrive there, and PCs are a rather diverse bunch of various D&D races. They come from a more advanced civilisation, so they have more advanced magic and equipment than anyone in the region, and they have their own system of hierarchies and aesthetics. They aren't however, any sort of mutant magical or otherwise, and they don't look like filthy barbarians. They're just different.

Obviously relationship between them and the PC and PCs home Empire would depend on PCs actions, but what I'm trying to figure out is what the initial reaction of a society that is built on the idea of superiority and conforming everything to its idea of hierarchy would be to the arrival of something that doesn't really fit into their idea of hierarchy, and strongly casts their superiority into doubt.

Well, not to be facetious but... what do you want to happen?

Even if we weigh multitudes of historical/fictional analogues and conclude that people in this faction should logically dive for pitchforks/torches/rifles or call in the national guard as soon as they spot your elf PC's pointy ears, ultimately you as the DM need to decide on a path that is fun for your players and allows the whole game thing to be able to progress.

My advice is to start from the fun, and then structure their reaction around that. Would your PCs be okay getting arrested and interrogated for a while as soon as they set foot in town, and could that be a useful device to impart exposition? If yes, that might be on the table. Would locking them up indefinitely or sentencing them to death rather than concluding that they aren't actually {insert object of faction's hate here} add anything to the campaign? Probably not, so don't do that; instead, have some authority figure conclude they're not mutants and let them go free. Said authority figure can even "benevolently" enlist them to help out with a little spot of purging he's been having trouble with.

TL;DR you don't really have to go full-on realism/history with this at all; generic autocratic behaviors/possibilities are fairly easy to predict.

[Kapow]

kill-the-heretic-burn-the-mutant types and are very focused on all sorts of things related to racial purity, hunting down people with magical mutations

If the PCs look similar enough, e.g. an Elf, a Gnome, a Dwarf and a Half-Orc, when this region is inhabited by humans, I think, they would be seen as mutants. Their technology/magic is obviously something they stole (from humans, or a trick)

[Mastikator]

Some options off the top of my head:

If the aliens are technologically and militarily superior then that means that they must follow some similar philosophy (at least in the uh fantasy evil faction's mind), so they'll seek some kind of partnership. At least until they have the upper hand to seize their partner's technological secrets for their own
These aliens are a threat to their way of life, maximum priority to eliminate them quickly. Mobilize forces against them, giving short respite to other factions
These so called "aliens" are nothing but a false flag operation by those evil and degenrate other factions and barbarians. Ignore them, they are fake news

[Anymage]

but they operate under the belief that their way is the only way to hold everything together and survive in their rather inhospitable environment, and that due to their discipline and order, they are the superior faction, and they do tend to call everyone who does not conform to their version of ordnung degenerate and [subhuman] barbarian filth.
So, fantasy nazis.

If they believe that their way of life is the only way to survive in their harsh conditions, technologically advanced aliens will most likely be viewed as a way to get an edge. (Assuming that the PCs are recognized as true outsiders and not as just extreme mutations.) First contact will most likely involve the highest ranked person on hand being tasked to interact with the PCs. As soon as word gets out to their superiors one of two things can happen, depending on how you want to spin it;
The extremists can want to take the PCs stuff through theft or force. In this case they're being set up as clear antagonists, and their evil ways should be played up through enslaving other cultures and/or exterminating them in an expansionist push. They're basically nazis in any action story about nazis, and exist primarily as antagonists to be thwarted.
Instead you could be angling for a culture clash/first contact type story. In this case the next step after the immediate reaction would be to sequester the PCs away from any sizeable population centers (you don't know if they carry any dangerous weapons, mutagenic properties, or even destabilizing ideas), and dealing primarily through diplomats as the extremists try to make sense of what's going on. There will be a lot of red tape since the pure will be polite yet insistent on maintaining a quarantine in at least the near term.

[ArmyOfOptimists]

"The enemy is both powerful and threatening, but weak and cowardly."

Appearance of an outside force that's actually advanced enough to prove a threat would be a boon to their ideals in the short term. Strangers would already be viewed with suspicion, but The Pure would most likely begin an intense propaganda campaign to paint them as cruel invaders here to stomp out all that the people hold dear, while also being easily defeated if everyone falls in line behind Dear Leader. Probably try to connect them to an existing group or nation - "They say they're from across the sea, but we know that's impossible. They're really working for Eatbabyland!" If the threat is real enough, they'd probably pause any ongoing oppression of existing outgroups and even try to recruit some of them into the fold. The fascist circle expands during a crisis of control and retracts once dominance is established.

Over a long timeline where the adventurers actually cause significant losses and liberate the population, they're more likely to turn on each other and collapse into factions and infighting as each seeks to secure what power they can before the eventual defeat.