Imagine humans of the present day and an enemy who suddenly appeared (seemingly) out of nowhere. The enemy has following capabilities:

Their army consist of all kinds of creatures from kind of humanoid, through more animal like, to weird tentacled monsters. But are led by inteligent beings.

Endless supply of troops. It can be anything like monsters emerging from portals, or some new spieces of mutated animals that reproduces rapidly, or just being created from thin air. It is not important how exactly this is done, important is that humans have no chance standing against this enemy in open combat since any losses to humans would be much harder to replace than for the enemy.

Widely spread and fast army. Enemy can march in groups widely spread to say 500km in width and at constant speed 200km/h. They dont need to eat and almost never rest. Combined with their endless numbers, this makes them hard to destroy by missiles and airstrikes.

Their are only superior on land. They do have aquatic, aerial and subterranean creatures but these are far weaker and less in numbers than the main army and can be defeated by human submarines/aeroplanes. This forces them to always move their army on land. They can get through rivers like night king in game of thrones used his zombies to fill the burning trench, but not huge rivers and not any sea.

They start with 3 armies in USA, arabian peninsula and Australia, where they immediately conquer everything giving them free access to almost whole North America, Asia, Africa, Europe and Australia. Initialy humans try to fight them in a regular way by deploying army, but they soon find out that it is absolutely inefective. Also the enemy leaders can comunicate with each other on the other side of the world, so they do cooperate.

Now, what I would suggest for humans to do is to create a heavily defended walls to guard places that are hardly accesible for enemy and evacuate there. This means mostly peninsulas or places surrounded by big lakes. And literaly every island in the world, since those are not possible for enemy to take. Humans would just need to guard the shores for possible attack from the sea, but those will never be as devastating as attack on the land. By looking at world map I found these places:

Srilanka, Indonesia(Oceania), New Zealand, Madagascar, Scandinavia, Canadian Islands, Quebec, whole South and Central america since you only need to build defense in Panama, Baja California, Mediterranean sea islands, Italy, Spain, Florida, Korea, Kamchatka, Yamalian peninsula, Greenland, Iceland, Britain, Japan, Hainan, Socotra.

Would it be possible to build these? Humans would have enough time to do this, because enemy spends most of his time raiding and destroying everyhing. But is there enough build material, is there enough companies and tools to to that? Would those places be sufficient to feed all 7 bilion people? Given that human face threat like this, they would of course cooperate at much higher level and put away all conficts and personal interests, but would that be enough? Or is there a better strategy to face enemy like this? Thanks a lot!!

Edit 1:

Hi. I dont realy want to go into details about the enemy, since they are pretty complex, but ill try to clear few things up. They can multiply in two ways and that is by simpy laying eggs and second by being spawned by the leaders. For the purpose of this question you can think of them as Zerg from Starcraft. And leaders being kind of brood queens and kind of warlocks from Warcraft. The more powerful the leader gets, the faster he can spawn. And the bigger the army is the faster it multiplies. The initial 3 armies are spawned in secret in the woods or sands and after being big enough, they coordinately attack all 3 at once. This secret spwaning is eventualy discovered by humans but when they start acting it is to late and the armies already count milions of creatures. Why they don't stop the wall building efforts? They simply dont know. No satelites no planes no nothing. So their stategy would be to destroy as much human infrastructure and production as possible. So when they conquer for example New York, they dont know that people started building a wall in Panama. This question in fact was not about how to fight the enemy. I was mostly about the posibilities of the proposed plan(building walls), so I added the reality-check tag. The question was more about how hard would that be for people. How fast can we build such walls. How many people would be needed to build and maintain such defense. Lets say that wall would have to be made from some hard metal, be at least 50m high and at least 5m thick. And most importantly if humanity can sustain itself in such conditions. Thanks

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    $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. You're asking a lot of questions on a service that's designed for a single, focused question per post. You appear to be asking for a reality-check (please read the tag wiki), but it feels like a high concept question and I'm not sure any answer can actually justify its position. (*continued*) $\endgroup$ – JBH May 19 '19 at 13:38
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    $\begingroup$ Your "enemy" is at odds with itself, being both (a) a coordinated attacker and (b) simply raiding. You can't have both (especially on this scale). What are the sizes of the starting armies? What is the rate of new enemy arrival? Is this a Pacific Rim type of thing? Most importantly, why is this question important? Worldbuilding (on-topic) is about the rules & systems of your world, storybuilding (off-topic) is about circumstances, action, and plot. This feels a lot like storybuilding, as the outcome is based on narrative necessity. $\endgroup$ – JBH May 19 '19 at 13:42
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    $\begingroup$ Africa and South America are technically islands, due to the Suez and Panama canals $\endgroup$ – nzaman May 19 '19 at 13:51
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    $\begingroup$ You have an enemy that is capable enough to portal-attack three locations simultaneously, but somehow cannot portal-attack islands? They can travel at 200 km/h, easily fast enough to defeat wall-building efforts, but squander that critical advantage by sitting around raiding? They roll over the armies of the world, and have infinite numbers, but are stopped by mere walls? None of these make sense. $\endgroup$ – user535733 May 19 '19 at 14:32
  • $\begingroup$ @JBH: reddit.com/r/WritingPrompts/comments/bqa482/… $\endgroup$ – DT Cooper May 19 '19 at 17:02

Eat the invaders.

Make the invaders taste delicious when cooked in a light butter sauce.

How convenient for us that they don't eat, and they are seemingly baffled by high walls. This will make corralling and containing herds of them much easier.

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    $\begingroup$ Use as source material for fertilizer or fuel might work as well. Maybe also for plastics. $\endgroup$ – Ville Niemi May 19 '19 at 15:28

Kill the head and the body will die.

I can think of a few ways.

  1. Kill the head.. They are led by intelligent creatures. Kill those creatures. Or cut off communications between creatures and troops. If not being driven to attack, these thing might not attack. They are not hungry, no reason to be angry. Maybe they will sit there.

  2. Get your own. Because ∞ - ∞ = 0. Find out how they are made and make some. Or co-opt one of the intelligent leaders and turn it against the others. Or hack their control mechanism and fake instructions from the leader.

  3. Choke points.. If you can cordon off or otherwise control spawn points, you can kill the monsters as they arrive. There is a trick like this for Plants vs Zombies on the last endless level.

  4. Disease. Microbes own this world. Maybe these monsters are susceptible? If they are not ghosts or robots and are made of meat, maybe some disease can spread thru them and take them out.

  5. Energy source. They don't eat. If this is zombie-lowth low scifi and you can hand wave where the energy to move is coming from, then this won't work. If you need to explain how they move, you can go after how they move. Which is going to be something interesting and possibly something humans tech can use too.

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    $\begingroup$ Infinity minus infinity only equals zero for infinities of the same size. My aleph-1 army will always best your aleph-0 one! $\endgroup$ – Joe Bloggs May 19 '19 at 13:47
  • $\begingroup$ 1,2,3 are impossible due to how enemy works. 4 should also be enemys advantage because they are the masters of bioengineering, but maybe that makes them a little overpowered. 5 they should get energy from some biological process that dont require eating(or maybe they can eat dirt sometimes) like breathing, photosyntesis etc. $\endgroup$ – user65070 May 19 '19 at 23:32

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