By the Emperor, I'm off for a bit to play a game and you again fill the thread with such insane amounts of absolutely wrong and other heresies. I don't even think I can reasonably reply to everything.
First, 40K is not a HFY setting. Just isn't.
Of course it is. HFY isn't about humanity being the best possible race, with best possible technology, with best possible everything, being best fucking Mary Sues, having everything given to you on a silver platter just because you're human, but about humanity fighting tooth and nail through immense odds and winning just because we don't give up. Warhammer humanity went through literally unimaginable piles of shit to still be the dominant race in Galaxy and has a real shot at actually surviving for long time yet. Warhammer humanity had to sacrifice so much, and the world seems so bleak and embody everything wrong about humanity, but the point of HFY is that there is nothing wrong about humanity. Through genocide, zealotry, ignorance, but also kindness, compassion and brilliance humanity continues to kick ass and take names, despite having literal Gods against them. Imperium of Man embodies Humanity Fuck Yeah, because they didn't have it easy, they had it the exact opposite, they were playing on the Hardestcore Ironman Permadeath Insanity mode with enemies having insane handicaps, and they're still going and they aren't going to stop, even if all hell comes loose upon them. Humanity Fuck Yeah is not about "heck yeah we're the best", it's about why we're the best. Which is fighting against all odds. It's about survival. And if there is one thing Warhammer humanity is good at, it's surviving. They should be, they're after like, few dozen events which threatened their complete extinction on Galactic scale.
Next, the Imperium may have a lot of mass behind them, but that doesn't translate to a linearly better force. They have systems and systems of resources, but are corrupt, selfish, zealous, incompetent, ignorant, stagnant, poorly managed, have poor communication, little centralized command, and little to no ability to actually replace any of their best stuff. Including space ships. You know, you get to the place where the fight is happening? They refuse to try advancing their technology or making new designs because grimdark. Humanity of 2018 has no such hangups.
The huge, untrained, uncoordinated brute with a sledgehammer does not win against a seasoned swordsman.
Calling humanity of 2018 a seasoned swordsman compared to humanity of 41000 is quite ironic. Imperium has systems and systems of resources, and are corrupt, selfish, zealous, incompetent, ignorant, stagnant, poorly managed, with poor communication and little centralized command, with little to no ability to actually replace any of their best stuff, just like modern day humanity. I bet in case of war with more advanced species which only real purpose would be making you believe in their god and pay tithe, humanity wouldn't be as united as they often are in movies. Zealotry is rampant in less developed parts of the world, and it has impacted the modern world in very serious manner (see, 9/11). Incompetence is also something that's often a thing, so is ignorance (Earth is flat so if you stand on the edge you can board Imperial spaceships), and humanity on global scale is extremely poorly managed and has anything but centralized command. The only real "advantage" is communication, but that's only because it's happening on planetary scale, while Imperium has to deal with interstellar communication, a feat at which they're also impossibly (as in, FTL communication by Psyker) better than we are currently.
The point of using nukes isn't to use them in ground combat. Ground combat is the very worst way to fight imperial forces. They'll just keep dropping bodies on you until you die, and if you survive that for long enough more elite units appear to punch your teeth in. No, nukes are for space combat.
Uh. Imperial ships slug nuke equivalents all the time, and have shields, interceptors and advanced point-defense. Whole supply of Earth's ICBMs might make a dent, but it's one use really.
What we're seeing here is a bunch of guardsmen being dropped onto a "primitive human splinter world", except they get defeated by tanks that actually work and infantry who use actual tactics. Once that initial ground force is defeated by the earthlings, the imperial navy has to order some replacement guardsmen. If that second wave arrives, the earthlings are pretty much doomed. But this is 40K grimdark verse we're talking about here, reinforcements take years to arrive. And while you're traveling through the warp, the earthlings are TRAINING.
Imperial tanks also work, and Imperial infantry, as weird is that is, also uses actual tactics. It's very easy to dismiss WH40k because it often focuses on Soviet-like strategies, and so on, but... the problem here is that Soviets won the war. Zhukov is regarded as the best commander of Second World War even by Western historians (a thing that never ceases to amaze me in the "how the fuck did that even happen) way, and he's the guy who literally said he uses infantry to clear out minefields. Even then, Imperium has ability to drop overwhelming forces anywhere on the planet, while Earth's forces would be spread out, incapable of any real interception and relatively slow to react. By the time US carrier groups get their shit together and armies across the world mobilize, Imperium controls White House, Kremlin, and literally any strategic location that allows them to dictate the war and disables Earth's nuclear arsenal. Even if the first wave fails, the Navy requesting the reinforcements probably won't be just orbiting around, they'd be shelling the fucking place to oblivion - they don't exactly care about environmental damage or even the local population.
They'll take every bit of IG tech they can get and study the crap out of it. Sort the useless barely-industrial crud out from the remaining glimmers of DAoT splendor. Scale it up, make it their own. That turns getting to space from hard and expensive to trivial. What did the guardsmen arrive in? Shuttles that can fly from orbit to the ground and back without refueling. We'll take those~
This is a very naive idea that implies Earth by itself has access to all the important stuff that's used in it. It would also require time which, unless you just outright think Imperium is absolutely useless (which I cannot really prove to you they aren't, because if you went that far you clearly are denying actual facts, so), they don't really have.
Now you have earthlings with easy access to space as well as nifty tech like efficient batteries upscaled from lasguns. We have a lot of nukes. A lot of them. Now we can put those in space for cheap. The imperium find it rather difficult to replace destroyed ships. It doesn't matter how many guardsmen or even space marines you cram into a ship if it takes 100 nukes
Which is essentially what regular space battles are in terms of Imperium, but whatever.
to the face the moment it jumps into the system. The imperium has countless bodies, mass produced armor, and mind-bogglingly deadly elite troops of a hundred different flavors. None of them can travel through space unaided. None of them will survive the ship they're riding disintegrating around them.
Of course they do. Marines survive stupid amounts of shit.
The obvious Achilles heel in 40k is the damn warp itself.
EVERYTHING about the warp is "It will kill you. Horribly." A tiny imbalance in the engine while traveling through it, and oops, you are all dead. A loss of hull integrity while traveling through it and whoops-- all dead.
All you really need to do is set up a sufficiently long-distance perimeter around your system, such that any ship trying to traverse the warp to get there will experience engine trouble from purposefully induced instabilities. (Instead of reverse engineering a warp drive, you instead produce a denial of service perimeter using the technology used to enter/exit the warp. You dont want to rip reality apart, you just want it to be sufficiently abnormal from what the imperium's navigators and engineers expect that they experience unpexpected irregularities in transit, and whoop, the warp has their ship.) They would have to exit far outside the system, in order to arrive safely. They of course, would never know that, because they would attempt to enter the system in dropship range, and the difference in distance traveled would be tiny for people still traversing the warp. (Less than human reaction time.) As such, they could never alert the imperium of the blockade devices.
Result- Imperium does not know why all the ships they send there never return. Does not know why none of them even report leaving the warp.
Still "We dont go to sol."
All you need to do is slightly alter the normal fabric of reality in the direct vicinity of your system, such that normal warp travel is impossible. For a highly isolationist world, this would be fine; They dont use warp travel anyway.
You can't. Humanity is non-psychic until like, quite a long time after, and while sure, Gellar Fields and other things can be operated without psykers, they're notably very power-hungry and have relatively small area of effect, and while there are various materials that work a bit like various blanks, they're rare even in terms of Imperium and again, don't work this way. Even if you had a giant fucking web of Gellar Fields around the Solar system, all it'd do is MAYBE drag Imperial Ships out of the Warp, but I don't think that's how it even works either, Warp is not equivalent to real space (except in Warp storms, kind of, still not really), which is why it's used as FTL and sometimes does stupid shit like time manipulation. You could probably somehow end inside that bubble without ever crossing the border, just like people end up in places they totally didn't want to go to, like Eye of Terror.
Yeah, I guess they could have said that their ships ran on prometheum or firewood instead.
Promethium is literally blanket term for all fuels. Some of them are advanced and are made from scooping up shit from stars, gas giants or so on, but it also includes regular diesel.
Also, any lore about why no one but Tyranids tries going extragalactic?
Navigation, mostly. It's fuckin' dark out there, and all the fluff implies that however weird and bad shit within known Imperial Space is, whatever is outside of it is probably worse.
Of course they do. According to Mechanicus tests, nearby void is filled with Orks. Silent King literally went on a voyage outside but he NOPE'd out when he found Tyranids on his way. Probably bunch more examples. It's somewhat rare, considering what it is, but yeah, it happens.
As far as humanity goes, I think /tg/ had this one fun thing, called "The Ship Moves".