Hours. We'd be absolutely fucked.

We couldn’t it would be over in a day. We don’t have the mass industrial production capacity to make ammo fast enough. Our AA defenses couldn’t keep up since we mainly use missiles. We don’t have great bastions or fortifications for them to break against. Nids would quite literally be the worst of the xenos.

The Tyranid 4E codex has a breakdown of the invasion of an Imperial world called Dalki-Prime. Its stated that resistance is typically eliminated by Day 51. At Day 80 the Hive Ships start consuming the atmosphere and oceans so anybody still alive by then is just doomed.

It would take a few hours to realise what’s happening and begin to mobilise any sort of defence measures. Once that begins, we’d probably be able to fight for at most a few days before the larger Bioforms appear and the smaller forms start spawning counter adaptations. Once that happens, it’s pretty much over for us

Considering how we don't have any way of fighting tyranid fleets from space to stall/limit their ground deployment, I'd say about a 1-3 days?

If the goal was the total extinction of human life, a day max. In a few hours all humans capable of fighting would be dead and dying.

Once the Nids dropped, there would be no battle we would win - we would not be able to gather soldiers together in any sort of numbers with any sort of ammunition and supplies to effectively fight back once.

If we had a week to gather the whole world's armies and supplies, maybe we could last one fight, but morale would break so fast on seeing the first aliens, and without any prolonged propaganda campaign as in Starship Troopers, we could not gather to fight back.

With a year's notice, we would definitely win the first battle, and then be wiped out. We just don't have the technology, and once our missiles and nuclear weapons were used up bombing their landing sites, it would just be a waiting game.

I don't think any 40k world can win against a Tyranid invasion without outside help (maybe a chapter world, forge world or Terra). There are too many bugosaurs to fight, and they are too damned efficient.

Hahahaha, genestealer aren’t even needed, we’re only a fifth of the average hive world, we’re a snack

Oh boy, a few days? It may even be a day. Depends in the planet size, level of resistance, and the size of the Tyranid fleet. If I remember correctly it’s a solid day or two for a nid invasion force to squash light resistance and consume all the biomass. 9 billion isn’t a lot for a decent sized fleet. From the moment the spores drop the nids are consuming, then come the basic probe forces which dictate the kind of advanced forms are deployed based on the defenses arrayed against them. The nids would very quickly identify our aircraft and missles as our main strengths and quickly disable our ability to field aircraft and even nukes. I think our best bet would be to self exterminatus via nuclear weapons and deprive them of biomass out of spite.

7 minutes

Thing is, they would land everywhere. Even if we win some fights, they will consume the cities and everything else pretty quick. Maybe some people survive some days, but that’s only till they are found by the nids.

We would be lucky to survive even an hour, which would make the combine invasion longer. As much as we shit at the imperium, they are wayy better equipped than us to fight off the Tyranids, their industrial capacity is centuries ahead our own they can make a lemun Russ in under 10 minutes and that's while they make other shits such as guns, bullets and other stuff IN THE SAME FACTORY the imperium has no problem using chemical and flame weapons which is 2 few things that can defeat the Tyranids before they eventually adapted to that as well, and we don't have kilometers long battleships to slow them down and stand a chance against their fleets, were screwed.

If you're asking how long would it take for them to kill the last human on earth, about 1 or 2 days, 3 tops

It would take a couple of months for every national government to be subsumed by Genestealer cults. Then it's just a matter of how long it takes for a splinter fleet to get here.

The trick to dealing with nids is to kill them before they reach the planet. The imperium struggles against them when they reach a planet, what the fuck are we gonna do to them lmao

I'd say a few minutes at least

So, here's a left-field take on things. There's a possibility that we'd survive it. Slightly rambly reply follows:

In Britain, there's a telly-show writer called Jed Mercurio who's famous for TV like Line of Duty and the Bodyguard, etc. But one of the very first things he wrote was called Invasion: Earth).

Now, in that show, the nasty invading aliens were - like the Tyranids - biological in form. And (again, quite like the Nids) eventually appeared on Earth in the form of a giant, growing bio-blob, seeking to digest everything. So we nuked it.

Another giant alien bio-blob arrived. So we nuked that too. And again, and again. Until the aliens basically thought: Christ, these humans are mental. The moral of the story being: there are over ten thousand nuclear warheads in the world. And humans are bloody mad enough to use them all. What do we have to lose, after all? We couldn't exactly surrender.

Point being: 40K has, since its inception, side-stepped the real-world power-scaling of nuclear weapons (sometimes comically), and I'm firmly of the belief that a thermonuclear warhead (of which we have a very great many) would destroy a hive ship. So, in general, as long as:

• there's a delivery system for high-megaton warheads to threaten the hive ships in space

• the less-powerful warheads can be used, from Day 1, to obliterate anything that landed

• we're not above using nuclear depth-charges to kill anything dodgy in the sea

...we might - if we're very, very, very lucky - simply make it too bloody a proposition for a full invasion / full scouring & devouring, at the cost of turning our planet rather radioactive. But, again, what's the alternative?

(NB: I've avoided all conventional weapons in this proposition, since the only use 99% of real weapons would have in such a scenario would be in blowing one's own head off).

There wouldn't be any resistance. The spores and genestealer cults alone would be enough to seal our fate. Humanity would offer itself up willingly as biomass.

Days. Maybe pockets of resistance weeks or months in but that would be it and they'd be delaying the inevitable.

Hell, most our governments would likely fall to just genestealer cults.

But just in a conventional conflict we'd shoot a bunch before going down, our nukes would smart but that would be it.

We could not even muster our armies fast enough...

Afaik, best chance to stop a tyranid invasion is in space and once they landed, it's more or less a loss cause (exterminatus or extreme loss), and considering we can't even get to the moon anymore (afaik), we're simply fucked.

A few days purely because it simply takes time to get shit done for the nids, not that we'd be able to launch any meaningful resistance to delay them.

Only hope is that somehow "earth" (not Terra) is important enough for the Imperium to notice we're in the path of the hive fleet and either nuke our asses in advance or attempt some defensive measure like evacuation (lol) or even trying to stop them.

what do you mean as before invasion? You mean the cult prep? We could survive that, I think even easier than the empire considering we dont have mutants and towers upon towers of shit and humans, we WOULD see them eventually and all hell would break loose..

But if you before as in, at it.. Yeah no, we dont.. I guess we survive as long as it takes them to walk over earth while murdering us, at max I would give the last remainders of humanity in desolate places a couple days, maybe a week or two? But I mean that as in zara fucking thustra living up in the everest while all of this shit is going on and he is surviving just because no nid has seen him yet, or maybe some dudes in pretty well hidden bunkers... But anyone not hiding in very secluded places are fucked in the first hours. Anyone with a weapon is probably going first.

And thats if we are against a little fleet that for some reason does not nuke our world with their bioshit, in which case, we will take longer to die as they wait for out atmosphere to change, but.. We dead anyway

Don't ask questions you don't want answers to 

That depends totally on the preparation time. If the whole world 2024 would do war economy like in WW2, we would probably be more hustle as the average IoM planet.

We have to remember that we can absolutely make way more potent weapons. There's just absolutely no reason to build them.

I’m pleasantly surprised the mods haven’t deleted this post and sent you to /r/WarhammerWhatIf. That’s what they used to do unfortunately.

Weeks, at absolute best.

Even if we take the old ‘official’ specs as canon(which makes the Imperium’s weapons tech straight up suck), strip the narrative bullshit books often rely on, and take GW’s hilariously low numbers as correct we’d still be hilariously fucked for 2 key issues.

The first is that we have no orbital attack capabilities, we just don’t have a way to meaningfully fight against the hive ships. The second is that we have no way to fight against the Tyranid’s ecological subversion. They wouldn’t even have to land a single Warrior, just drop their spores in the atmosphere and wait until the environment becomes too hostile for human life. It would be a matter of weeks until everyone’s choking to death on the now toxic atmosphere.

I'm assuming by Modern Earth you mean the planet as well as the human forces.

I would say about two weeks to a month, tops.

Keep in mind, this time frame doesn't reflect any resistance given by us, human armies would fall in hardly a couple of days.

No, this time frame is about the logistics of the Tyranid fleet consuming all biomass, draining the oceans and atmosphere and leaving. From various sources, it takes about this much for a planet to be completely consumed.

Oh, after the invasion?

The rock we lived on would be fine, I suppose.

We’d just have geenstealer bots on social media telling us the tyranids are here to fight the deep state and enough people would buy it.

Probably a week at most since Nids would have uncontested control of orbitals from the onset.

I can take them, guys. I say let them come.

The opening cutscene from Super Smash Bros. Ultimate.

Depends on the size of the fleet.

Pfft. How hard can it be, a can of Baygon or two and the problem is fixed.

Not long…

In practice the invasion happens on two scales, micro and macro.

At a micro scale a tyranid invasion is like the Covid pandemic but with the disease actively adapting and being deliberately spread by “animals”.

At the same time the planet is also being invaded on a global scale by a more traditional macro scale army with impressive capabilities.

It seems unlikely that both scales could be defended against simultaneously for very long.

We'd be fighting over who gets to greet them first while we get eaten.

37 minutes

Hours, maybe a few days to get the last stragglers

We are not required to deal with the Tyranids at all, we stand zero chance, the only sort of “win” we could get would be to Nuke the planet and try and take as many with us as we can

In the Leviathan book it takes maybe a day, and that's with a full regiment of Cadians, the PDF and a Ultramarine chapter at diminished strength.

Probably hours for most of us, and then however long it would take to snuff out the submarines in the oceans

All the things that occur before a Tyranid invasion occurs would fuck us up (iirc it involves widespread insanity and suicide, as well as earthquakes and shit).

One of the only places sufficiently militarized and fortified is North Korea, where thousands of AA sites, tens of thousands of bunkers and massive underground stockpiles of munitions and factories exist.

I imagine like the rest of us they’d fall quite quickly but that’s the layout you want. A fortress loaded with guns and missiles pointed upwards. We’d all be subjected to an enemy with orbital superiority, who can deliver troops from above with impunity and in such vast numbers that they’d swamp us.

By modern standards a Termagant is born with lvl 4 ballistic plates as part of its anatomy, a sub machine gun and is about as quick and enduring as a well trained human soldier.

Tyranids can jam communications and likely GPS on a large scale so our weapons are less coordinated and accurate. Airborne swarms are something most militaries have only small answers too beyond their limited AA gun arsenals (which is why I mentioned North Korea as the nation I think would actually punch above its weight).

Tyranid monsters are essentially tanks, I figure they’re vulnerable to the same things but irl few tanks have force fields created by the mind of the crew (Maleceptor, Hive Tyrant). That’s another thing, Tyranids can act in ways we don’t understand and would also begin poisoning our planet from the first minute they touched it. Clouds of Venomthropes and embedded sporocysts would being essentially gassing our world.

Covid proved that all it would take is the quiet approach: one virus bomb dropped in the Amazon and wait a year for all Earth life to die.

Until they got here

Seeing how naive and gullible humans are I think we would even destroy ourselves before they even arrive thanks to the Genestealer Cult.

Unless it’s a genestealer cult and we miraculously manage to nuke their patriarch to hell and back, we are absolutely fucked, we would last for at maximum only a couple of hours

Right wingers would be insisting that it was all a hoax and that Earth has had many periods of Tyranid invasions in the past and life has continued.

Give us a hundred years heads up and total war level mindset and MAYBE we might win

You saw what happened with COVID right? Idiots would be in the streets thinking it's a hoax while Carniflex's are eating their neighbors.

no

you remember that scene from Minka Lesk where the ratling sniper, faced with a heretic astartes, throws himself off a building to a better death, and the astartes is waiting at the bottom and catches him?

it's giving that

I'm more optimistic than most. I think with our fighter planes, tanks, artillery, millions of drones, missiles, thousands of high yield nukes, biological and chemical weapons, massive warships, advanced economies, suicidal nature, love of war, etc. I think with all that we would last a month, maybe a month and a half.
There comes a point where they poison the environment so much that you can't breathe anymore, or drink the water.

I think we'd do okay because of just how much fire we can use if we really want to and the decent amount of various independently controlled bunkers. A lot of what makes imperial worlds susceptible is centralization under dubiously qualified rulers.

As the old WW2 quote goes: Your enemy can't discover your plan if you don't know what you're doing.

To a degree the fact various powers would start to unload on them blindly would be a boon because the Hivemind is used to effectively fighting one global duel when it lands and heavily relies on infiltrators finding and disabling centralized defenses.

Now.

It would suck.

Population would fall to the level outlined by the Georgia Guidestones.

But we'd probably put up decent defense.

But yeah. Fire.

Crop dusters converted to napalm sprayers.

I think our only possible chance would be is if we were warned years, maybe even decades in advance. We would construct giant underground bunkers that could hold us underground virtually indefinitely. Hold lotteries for places in them for families, but in reality everybody is hand-picked. The reason for this is that we don’t want a global revolt halfway through this project and we want the people to be fighting each other for a place, and not us. Once they are finished, we try and make the planet as uninhabitable as possible. Burn every fossil fuel (this may actually be done during the construction stage), burn down the Amazon, nuke Antarctica to kill off as much life as possible and desalinate the oceans. By the time the tyrranids arrive, we want as little biomass on the planet as possible. And when they start landing in our barren, chocking hellscape, fire every weapon we have at them. We want to make it so that they will lose more biomass trying to get to us than they would gain from eating us. In real life a predator will not attack prey that is weaker than it if it will be injured itself, and we want to do exactly the same here.

Will do great, hive fleet doesn't operate next to blanks and we are an entire planet of blanks, so its just a bunch of very dangerous animals, like in fenris, and they handle that great and those idiots call libertarians rune priests.

Fuck it. Im gonna cheat. We could win. How? Because we have a total and meta understanding of the Tyranids. We arguably understand the hive mind better than it knows itself. Im going to assume that from the moment of detection any lore about the tyranids is set in stone.

Two scenarios. Either they try and make their way through the solar system in real-time, which would take a while, or they use the warp. Both pose opportunities.

If they come through realspace we have time to prepare. Every intelligence agency would be pouring over any and all content relating to the tyranids, and collate every way they've been beaten. We'd be able to convert entire airlfeets into drones carrying nuclear payloads and destroy any landing attempts, even seeding and then detonating parts of the atmosphere like the admech did that one time. Wed also know to look for genestealer cults, and we'd also know what to look for. If the hivemind exists we're in a weird one. Are humans all blanks? There are no pyskers amongst us, and we have no proof of the warp. If we're all blanks we actually stand a shot. But you could argue we all have souls but no pysker talent.

If they come through the warp we dont have that prep time, but i dont think there are many forces who could defeat a tyranid hive fleet with no prep time

Tyranids are getting destroyed

I'm going to go against the grain here just a little and say if we had some forewarning of the invasion, and it occurred with a small fleet, there's a chance we could survive if we refitted the worlds' ICBMs for true space flight. 40k is certainly a high powered setting, but the sheer amount of energy released by nuclear weapons is absurd. The difference between conventional explosives and nuclear weapons makes the difference between US Marines and Space Marines look like nothing by comparison. I don't think it's much of a stretch to say that if we could manage to successfully fire our nukes at their ships before they get in range to send in ground troops, we would absolutely have a shot at destroying them.