I WILL PREFACE THIS BY SAYING: I'm not trying to punish my players for anything whatsoever, I'm trying to create a feeling in order to immerse them and I into the world. I'm also not trying to make everything serious or doom and gloom, just trying to show that while yes, the world can be a fun and beautiful place, horrible things also happen and just being a bard, druid, wizard, etc. doesn't make you immune to the atrocities of war.

I'm writing my first campaign, and I already know who the players will be, I've played with them all quite a bit, and I'm pretty close friends to them all. A few of them are seasoned DnD veterans, but a few of them are brand new to TTRPG's. These new players and I are currently players in another campaign ran by our house DM, so I see how they play their characters (mostly based on their own personalities). The thing that worries me about DMing them is that they all have a really bad case of "Avengers syndrome". Even at level 1 they RP as if they're the saviors of the universe, all powerful, invincible, "destroy the city but it's cool someone else will clean it up and nobody will be affected". They treat it like they're playing Skyrim or something like that, where nothing has any real consequences. They can be pushy/rude to NPC's, rob/kill NPC's that didn't need to die, and basically act as if they've beaten the BBEG before they've even met him.

Now, I understand that sometimes their character might be an egotistical "Gaston" type of character, and I don't want to stifle any fun, but in THIS PARTICULAR campaign I want the players to understand and feel  the consequences of their actions in-game, and realize that the NPC's are people too, not just AI. And that they're not heroes until they've earned that title. I definitely write more serious "reality but with fantasy elements" type of campaigns/stories.

So the campaign that I'm writing could best be described as "WW2 Europe but with magic and martial weapons instead of guns, set in a fantasy world". The players start in what could be equated to Poland right before Germany invades. There'll be a few sessions before the invasion, so that they can form emotional attachments to NPC's and the city. When the invasion DOES happen, I want to formulate some sort of unbeatable enemy to create a sense of helplessness. I think of it like: If you're in a tornado or in an air raid/bombing, you're not gonna be able to stop it no matter how tough you are. But I know my players, I know that they'll try to go fight whatever it is that is attacking. Obviously I don't want to kill off any PC's in the 3rd or 4th session, but I also want them to have a "holy crap, I'm not invincible, this is REAL, I could die here" reaction to it. I've considered sending a super strong NPC out there to try to face it, only to get wrecked, but it's also just an NPC.

So basically, I'm stuck. I want them to get a real sense of the danger in the world, and I want them to have an emotional response to it and rise to the occasion and become the heroes, without an NPC tasking them with it, without a real quest. I want them to feel fear so that they actually have to be courageous, and have a real cause to fight for. Not just fight because they're the PC's. BUT, I have no clue how to do this without bullying the PC's. Also have no clue what type of beast or weapon to use to give them that feeling. 

 I think of it like: If you're in a tornado or in an air raid/bombing, you're not gonna be able to stop it no matter how tough you are. But I know my players, I know that they'll try to go fight whatever it is that is attacking. Obviously I don't want to kill off any PC's in the 3rd or 4th session, but I also want them to have a "holy crap, I'm not invincible, this is REAL, I could die here" reaction to it. I've considered sending a super strong NPC out there to try to face it, only to get wrecked, but it's also just an NPC.

You've basically given yourself the answer right here. Put the party in a situation where there is no enemy to fight and all they can do is try to save themselves or as many people as possible.

Earthquake, volcano, sink hole, biological weapon, etc. The common trope is that the "Good Guys" arrive right before the "Villain" pushes the "Big Red Button", and manages to save the day at the very last second. However, sometimes the Villain doesn't bother with the long winded victory speech and instead just pushes the button. By the time the "Good Guys" arrive, the "Villain" no longer cares.

For example, have the party hear an explosion a few blocks away, and when they go to investigate, let them see people running, stumbling, and coughing their lungs out onto the street as they die, screaming in pain. Then, ask the players to roll a constitution check... If anyone fails, describe them coughing blood into their hands. Something mild, but concerning. Then, every few "rounds", have them roll another one at a slightly higher DC. The longer they spend in the fallout zone, the more severe the symptoms get. They need to get out, and they need to find treatment fast. Let there be more valuable NPCs than they can save. Do they prioritize the Magic Item dealer, the powerful wizard ally who was knocked unconscious by the blast, or the huddled families hiding in their homes?

Someone is going to be left behind to die, and they'll be responsible for choosing who. Their choices will have both mechanical and narrative consequences.

1. Saving the families will make them local heroes, but they'll lose precious NPC resources.
2. Saving either the powerful wizard or the Magic Items Dealer will give them a valuable ally, but the city will become a ghost town. A monument to their failure.

Maybe a survivor will see them abandoning the civilian NPCs in favor of the Magic Arms Dealer, and will spread word to other towns of their apparently unethical behavior.

You could then add depth by making them choose between different groups of NPCs. Typical Trolley dilemma.

You could give yourself a few categories, and score them accordingly: Exposure | Reputation | Assets

• For each point in "Exposure" (Gained by staying too long), they take points of Exhaustion that take longer than normal to heal. Maybe even semi-permanent wounds.
• For each point in "Reputation" (Gained by rescuing narratively important people/things), they gain bonuses to Charisma checks in the region.
• For each point of "Assets" (Gained by rescuing mechanically important people/things), they gain material boons.

If the party scores low in "Reputation", but high in "Assets", some of those assets might abandon the players after rewarding them, due to survivor's guilt.

If the party scores high in both "Reputation" and "Assets", they'll probably suffer some pretty severe "Exposure", but could gain long term benefits, like a patron or a sponsorship. (This should be a deadly outcome. A player might decide to martyr themselves, which could become a highlight of the game, so long as the players are sufficiently warned of the risks ahead of time. Player Autonomy is paramount. Breaking immersion to brief the players before the session starts is entirely warranted.)

 Later on, you could have them encounter a False Hydra.

One thing that you can do is put your DM screen away and run a deadly combat for their level with all of your rolls out in the open. Especially at lower levels that will make a large difference as you won’t be able to soften any rolls and one unlucky die roll can easily drop a PC to 0 HP. 

The thing that I’ve done with players who play as if their actions don’t have any consequences is I think about what would happen to me if I tried to do what they’re doing in the real world. It’s definitely a wake up call when a PC says, “I do (something rude)” and the DM says, “The entire room goes silent and you can hear gasps. Almost as one everyone backs away from you and you see....” Then if they win the half ogre or table of city guards or 10 crossbow wielding commoners they “get” to see drawings of their faces on wanted posters and hear songs about the cowardly thugs who (did something bad) and they’re described in the songs to the point where someone in the crowd around the minstrel says, “There they are!” and points to them. 

Even if they don’t do anything bad enough to deserve to be beaten up, they can find that suddenly the taverns won’t serve them and the inns don’t have any rooms available if they misbehave  

After all, that’s what would happen to us in the real world if we act the way you’re describing. Just be realistic. 

This might sound like something obvious but have you actually told them that they need to level their expectations?

When I explained to my players how my campaign is going to look like, one of the first thing I've told them is that it's not a world of dynamic scaling to their level - there are some areas where monsters are way beyond what they are capable of. And I told them honestly "guys, just because something exists there, it doesn't mean that I expect you to fight it and win it".

And once they know it, they are super careful, sometimes debating to the death whether they will be able to handle a situation or not. 

Same goes with mysteries - they have found a journal in which an ancient archmage describes a puzzle she was attempting to solve. Later they found her abandoned sanctuary. And they know, because they got told, that sometimes just because you find a riddle element, it doesn't mean you are supposed to solve it right away. If an epic level archmage couldn't solve it, don't expect your 6th level novice ass to go in and save the day. Just keep it in mind for later, perhaps more clues will appear as the story progresses.

Quote from Fuzzy_Pinetree >>

So the campaign that I'm writing could best be described as "WW2 Europe but with magic and martial weapons instead of guns, set in a fantasy world". The players start in what could be equated to Poland right before Germany invades. There'll be a few sessions before the invasion, so that they can form emotional attachments to NPC's and the city. When the invasion DOES happen, I want to formulate some sort of unbeatable enemy to create a sense of helplessness. I think of it like: If you're in a tornado or in an air raid/bombing, you're not gonna be able to stop it no matter how tough you are. But I know my players, I know that they'll try to go fight whatever it is that is attacking. Obviously I don't want to kill off any PC's in the 3rd or 4th session, but I also want them to have a "holy crap, I'm not invincible, this is REAL, I could die here" reaction to it. I've considered sending a super strong NPC out there to try to face it, only to get wrecked, but it's also just an NPC.

 You can always just make it so there isn't really anything to attack; just swat the area the PCs are in with a long range spell of mass destruction that won't necessarily kill them outright, such as Earthquake or Storm of Vengeance (or Meteor Swarm an area they're not in). There's not much they can do about it, the spellcaster is out of range of any spells they have even if they know where he is and they might be wary of going after level 15+ spellcasters, but it probably won't actually kill them.

It's not just your players. Most players really, really hate to run from a situation. People have been trained both by other media like video games in particular, and also by the game design in D&D, to think there should always be a way to win every encounter, so running is a kind of failure. 

I agree with Lathlear that you should warn them as part of your session 0, their characters could die, they won't be able to win every fight, and their will alwys be consequences for their actions. Then follow through. I'm not saying you should set out to kill characters, because that's just bad form. But after they've been warned, don't pull punches and don't be afraid to put them in a difficult situation. If they remember your warning and flee, they live -- and there's some consequences depending on just how they do it, like memnosyne's ideas. Maybe a few die, and then you have the rest to go on with the campaign. If they stay and tpk, well, it happens sometimes. You can then start a new campaign, and I'd say even do it in the same town, a few months later, when the populace is now under occupation. And the original party is remembered not as heroes, but as fools who rushed to their deaths leaving everyone else to fend for themselves. And probably other consequences, based on what they do as they fight.

Even at level 1 they RP as if they're the saviors of the universe, all powerful, invincible, "destroy the city but it's cool someone else will clean it up and nobody will be affected". They treat it like they're playing Skyrim or something like that, where nothing has any real consequences. They can be pushy/rude to NPC's, rob/kill NPC's that didn't need to die, and basically act as if they've beaten the BBEG before they've even met him.

There is no DMing technique in the world that can fix this. This is an OOC player issue, not an in-game issue, so no plot, no NPC dialogue, no battle, no in-character anything, can affect this behavior.

You might, perhaps, have some success talking to them OOC, and explaining to them that there is a different mindset you are trying to achieve in this campaign. But you will not be able to get your players to "RP differently" just by giving them any sort of in-character scenario.

I suspect with the group as you describe, it is going to be very hard for you to achieve the dramatic tension you are trying to go for. You might be better off giving in to what the players want to do and having more of a "gonzo" game where they really are the avengers and can take on anything and possibly win (or at least not auto-die). You are not going to be able to make them RP the way you want them to. 

I know, you didn't say that's what you want but, realistically, based on the OP... that is what you want: for them to RP in a more serious, long-game kind of way, than an "I AM IRONMAN" way.

Give them an NPC that they adore and respect, and then have your big bad or monster or whatever kill that NPC easily. Now they're afraid of the monster AND mad at it for killing a friend. If they still aren't afraid of it and try to fight it, well, death does happen in the world.

So it was only said once, while everyone else is contributing ideas for wrecking towns.  But it needs to be said again, for emphasis.

Tell.  Your.  Players.  That their usual behavior will get them killed.  Tell them, from the start and in no uncertain words, that your world is different and there will be no second chances or DM fiat.  If they come at this story with the idea that they're the most important things in it and the only people that matter, they will die humiliating deaths and no one will mourn them.  If you want to try and run this game, tell your players up front.  

Then.  Gauge their reaction.  Do they look jazzed at trying something new, engaging with a different type of narrative.  Or do they look frustrated, pissed off, or depressed?  Much as DMs like it, not every player is down for the gritty, earn-your-Hero-ending campaign.  Some players like the casual beer-and-pretzels power fantasy, that's their jam and they're playing to get that high, and there's nothing wrong with them for liking their D&D more Skyrim than Grave of the Fireflies.  In point of fact, there's plenty of people who will abandon ship and never play a game with you again if you drop Grave of the Fireflies on them without any warning.

Session Zero is extra important when you're thinking of radically altering the tone a gaming group is used to.  Ensure your players are ready for this shit.  Don't try and dick punch them with it out of nowhere in the campaign; ensure they're on board with the more serious tone from the start.
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...then dick punch them with a brutal-ass invasion.  I recommend asking the players to associate themselves with a local Captain of their nation's guard.  A relatively heroic, likeable sort who's friendly and personable without being a carpet, the sort of compassionate leader that's immensely popular.  Set him up as a possible taskmaster for the group, a patron of sorts.  A powerful heroic figure they can aspire to.  Then, three or four sessions in, when the invasion starts, have him take command of the retreat.  Organize things, direct people, be all heroic and shit.  Just long enough to drive home that this man is excellent at his job, before he dies in a horrific and irreversible manner.  If you can pull it off, make the players feel like this guy is their ally in the world and their support from On High, watching him get torn apart by wargs or whatever you end up picking will be a dagger in the gonads.

Give them an NPC that they adore and respect, and then have your big bad or monster or whatever kill that NPC easily. Now they're afraid of the monster AND mad at it for killing a friend. If they still aren't afraid of it and try to fight it, well, death does happen in the world.

Again... this is not going to work if OOC, the players want to RP being bad-a$$ Avengers types who are punching Thanos in the face without fear and they do not want to RP the more gritty-realistic Cthulhu style "oh crap run for your life! We're only 1st level!" type of story.

We cannot solve an OOC situation with IC conditions -- it will not work. Ever. It may seem like it will work, but it only seems that way because we, as the DM or the person with the issue, are already, OOC, wanting to go in that direction. A bunch of players who just want to murder-hobo through a dungeon or who want to kill NPCs and take their loot, will not respond positively to the death of a "beloved NPC". And by "not positively," I am not talking about their PCs mourning the NPC and fearing the big bad. I mean they will not RP in the way the GM is hoping or anticipating. Instead they will hate it, and probably react badly, and not at all in the way the DM is planning.

So talk to the players, and find out what kind of game they like. If they don't want a serious/consequences/we-are-scared-newbie-characters type of play, don't try to force it. If you are not, as a DM, happy with the more gonzo "Avengers RP" style these players like (I'll be honest -- I would not either), then get someone else to DM. Do not try to force your style of RP onto players who don't like it.

Avengers or not, every PC needs to know when to run. I’d start off with scenarios and simpler battles that train them to read the battlefield better.  

Scenario 1:  The Front Lines.  On round 1 the party is ambushed by 2 goblins.  Round 2:  1 more goblin.  Round 3: 2 more goblins.  Round 4: 3 more goblins.  Each round, you add a linear number of goblins with the intention to show them that the front line is getting closer and closer.

Scenario 2:  Swarms.  For one battle, have a large number of low-CR creatures target ONE and only ONE PC.  Every single one.  Force the team to do whatever they can to help this person because they’re getting overwhelmed each round.  Play the creatures mindlessly, not strategically - they will incur opp attacks just to get to their target. 

These are two ways to humble your low level characters AND teach them about tactics as well as gauging the battle.  It moves them away from playing solitary heroes to a tight knit group.  It lets you roleplay an Orc leader later on that says “Focus fire on the wizard!” and the party *notices* it.  It just creates awareness on the battlefield.

Ive used these to great effect in teaching new players the ins and outs of battle, I hope they help you too!

I really like Memnosyne's suggestion... it's just got a lot of built in drama and really challenges the characters without just saying, "here's a big dude in a fight you can't win, now either he's gonna kick your ass or I'm going to introduce a cool NPC for him to kill in front of you".

That said... I'm curious about how players might try to make a Cool, Powerful NPC that the players would actually like. It's hard to pull off... a lot of times these powerful characters end up feeling like they're stealing the spotlight, or they just come off as a DMPC power fantasy. Anyway, just thought I'd try to steer the conversation in that direction because I'm hoping for more advice on how to pull that off.

That ultimatum idea is solid! I like that they have to make a choice, both could be considered "selfless" but if they pick the more selfish one (the NPC resources) results in lower reputation or a ghost town.

"The common trope is that the "Good Guys" arrive right before the "Villain" pushes the "Big Red Button", and manages to save the day at the very last second. However, sometimes the Villain doesn't bother with the long winded victory speech and instead just pushes the button. By the time the "Good Guys" arrive, the "Villain" no longer cares"  This is what I was trying to put into words but couldn't get it straight in my head.

"RP in a more serious, long-game kind of way, than an "I AM IRONMAN" way."  This is the key issue I think. None of them (nor I) have ever been in a campaign that lasted more than 6 months or so. So they don't have that realization that campaigns aren't like a regular game where there's a beginning and an end that takes place within just a few months or so. I've gotten well acquainted with the long-play aspect of it through worldbuilding and story writing, but I can see how if you expect a campaign to exist in a bubble over a short time that you'd be more inclined to charge in gung ho. 

Is the real issue with getting the PCs to run, or is it the "They can be pushy/rude to NPC's, rob/kill NPC's that didn't need to die, and basically act as if they've beaten the BBEG before they've even met him." part?

Both sort of. The real issue is hubris I guess. The sense that "We're the heroes, everything will work out for us in the end, and we're the most important people in this world". So they don't run from fights cause "it's part of the game, we must be able to beat this" and "it doesn't matter if I'm super rude to this NPC, if he's a key part of the story/quest progression then he'll have to help us either way".

Both sort of. The real issue is hubris I guess. The sense that "We're the heroes, everything will work out for us in the end, and we're the most important people in this world". So they don't run from fights cause "it's part of the game, we must be able to beat this" and "it doesn't matter if I'm super rude to this NPC, if he's a key part of the story/quest progression then he'll have to help us either way".

Here again... if this is what they are used to, and if they like it, you can only change it at your own peril. If they want a game in which they are the most important people in the world, everything works out in the end, and if this NPC is a key to the plot he will have to help them in the end, then you either need to provide that, or have someone else be the GM.

You need to have a conversation with your players. Lay this out on the table. Tell them that you are envisioning a longer-game, serious, consequence-filled campaign where if they kill off the only NPC who had the key to solving a quest, that quest literally cannot be solved, and this is not a video game in which they can save and reload. Are they not just willing, but eager, to play in a game like this? If they want it, great. If they say no, they'd rather have it the first way, and you are not willing to GM that, then say so, up front. Not as a threat! ("Play it my way or else!"), but as information to them, and ask if someone else would like to DM or maybe they want to do rotating DMs.

These players clearly have the "video game mentality" (you mentioned this in the OP). The question is, do they want it like this? Or are they looking for "something more?" You need to ascertain their tastes before you try to force a game play style on them that they dislike.

Quote from BioWizard >>

 The sense that "We're the heroes, everything will work out for us in the end, and we're the most important people in this world". So they don't run from fights cause "it's part of the game, we must be able to beat this" and "it doesn't matter if I'm super rude to this NPC, if he's a key part of the story/quest progression then he'll have to help us either way".

 

These players clearly have the "video game mentality" (you mentioned this in the OP). The question is, do they want it like this? Or are they looking for "something more?" You need to ascertain their tastes before you try to force a game play style on them that they dislike.

To piggy back this:  There's nothing inherently "Wrong" with players wanting to play DND in a way where they are the heroes of the action movie.  If that's what's fun, that's what's fun.  There is no rule that says that they have to be afraid of fights, or want to get to know the backstory of every NPC.  So when you talk to your players, be mindful that the end goal of all of the game is to have fun at the table, be it as Iron Man and Hawkeye or as a party of Grim Dark Adventurers.  Ditto for the NPC's.

And of course, make sure you know how to articulate what YOU find fun.  If you find the whole NPC creation thing fun, and your players are getting into being "super heroes with attitude" then you're going to have conflicts in what you find fun and given that DM'ing takes a lot more planning than playing... it's not a good recipe.  

But that loops back to:  Talk.  To. The.  Players.

I didn't mean that it's "wrong" for them to play like that, I just wanted to try a campaign with a different vibe than what we usually do. That's why I posted this was to find a way to do that without stifling their fun, and it seems that communication is key to it all. 

But Yeah I'm talking to them about it for sure, and if they really aren't down for it then I'll save that sort of campaign for later. I've talked to a few of them and they dig it! 

The other problem is that, as a DM, you want your PCs to do the adventure, so if they go about doing things that plausibly should leave them incapable, there's a tendency to try and find a way for them to do it anyway.