What would out-of-water warfare look like for underwater people?

Question

In my short story, there are underwater people who are now up to WWII technology-ish (major differences, but that level and population of society). These mermen can hold their breath for as long as we can. There are no out-of-water humans, but different rival societies are vying for the precious resources on land. As early as 100 years ago, they've learned how to extract just like we've learned to extract from under the sea.

For control of resources out-of-water (flora, fauna: "fishing for deer," more mineral extraction), how would they dominate the lands?

Slightly related: Weapons for Mer-people.

Answer 1

To answer this question, it is important to note that while humans can operate underwater, they prefer to do most things on land. We've been built by evolution to do best on land, and thus trying to work underwater usually turns out to put us at a disadvantage. This is probably going to be similar for merfolk; they're built for water, so doing anything on land is going to be hard for them. They may be able to invent things that help them survive on land, but at the end of the day they'll be glad to jump back into the oceans.

That said, the merfolk may find that there are some perks to working in the fresh air. For one thing, there isn't much resistance; this is bad for swimming, but good for things like weapons. Swords will swing faster, and arrows will fly further. I don't imagine the merfolk will have invented guns by this time, as guns tend not to work too well underwater, but to make up for this I'm sure they'll have many highly advanced underwater weapons that should work pretty well on land (and if they don't, like I said, bows are going to seem incredibly powerful to them without water resistance slowing down the projectiles).

Something that tends to not be as dangerous on land is explosives. If you're underwater and a grenade goes off next to you, even if you don't get hit by the schrapnel it's the shockwaves that'll do you in. On land, on the other hand, shockwaves in air don't do a whole lot. Based on this, it's plausible that merfolk won't deal too often with explosives on land. They just won't have the technology for it. If they discover fire, though, that's going to catch on like, well, wildfire. It's something that they can't use underwater, but is greatly effective on land (especially if they're deep-sea merfolk who aren't used to light).

From this, we have merfolk fighting with fire and simple projectile weapons (let's say crossbows). Now, as for where they'll fight and with what tactics, I'd say they will mainly be focused on getting back into water. As I said earlier, they aren't going to like being on land. But more than that, they probably can't even survive on land for too long. Humans breathe oxygen, and oxygen can be compressed to offer us a lot of air to breathe underwater; merfolk, on the other hand, breathe water, a mostly uncompressible liquid. Thus, their maximum water supply will be a fraction of our maximum air supply, especially since without water resistance anything the merfolk carry is going to feel a lot heavier than usual.

So I can see land combat revolving around bodies of water, with suited-up soldiers floundering on land from pond to pond, trading bolts and molotovs. Something similar to trench warfare may develop, as both sides build irrigation trenches through which they can swim. The no-man's-land will become the land, and anyone who tries to cross it will find themselves an easy target.

Answer 2

The same way we dominate the oceans. Think about a submarine: From the outside, it's a vehicle made for going through water, while in the inside it's a container for air, and has everything humans need to survive for a while.

Similarly those merfolk people would build vehicles filled with salt water. I don't think they would build airplanes because those would simply be too heavy when filled with water (and WWII technology isn't yet ready for drones). Civil vehicles would have arms similar to the arms of research submarines. Military vehicles might be water-filled tanks.

The main problem would be that those vehicles have to be heavy, first because of the water, and second because they need to be tight enough to keep the water inside (and plastics is not yet available at WWII technological level). Therefore even the civil vehicles would likely resemble tanks, just that instead of weapons they have manipulator arms. This restriction will likely also restrict the uses of such vehicles (just like our use of submarines is limited).

I think the most common civil vehicle would be a transport vehicle, allowing to transport things over land where the sea route would be much longer or even doesn't exist (if oceans are completely separated by land masses, similar to how our continents are separated by oceans). They would basically have the same function as our ships.

Note also that they may use rivers as way to get far into the land before they have to leave the water (assuming they can survive non-salty water, that is; but even if not, for them making a salt water-filled underwater vehicle would probably be easier than to make a land vehicle, not to mention that those river vehicles would have less issues with weight).