How do I effectively wage war against an aquatic alien species?

Question

I am revisiting an older scenario I came up with, but I have been experiencing many of the same problems. So this question is similar to this one, but with some important differences.

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Two planets rotate each other and are tidally locked. They are where earth is around a star nearly identical to the sun. One planet is much wetter, featuring 85% oceans, wiscattered swamps, jungles, and the ccasional forest. The other is much dryer with plains and deserts. Both are about earth-sized and have similar atmospheres. They are close, about 16550 miles center to center. A moon about the size of ours orbits the two. This system orbits a sun at he same rate as earth. On one lives a race of lizard-men and on the other lives a race of eel-men. I'll let you pick which species belongs on which planet. Anyway, they are both extremely sensitive to radiation. Other than that, you can pretend they are human.

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Both planets have futuristic technology, so they have all the modern conveniences of earth, and more besides. Except no internet or social media. They do have, however, large unwieldy inaccurate plasma weapons, ineffective laser guns, and good old projectile weapons like we have. (You know, all of the weaponry we have in modern times on earth.)

The lizard people issue an ultimatum. The inhabitants of the other planet must evacuate to their space colony or face all out war, and be subjugated as slaves. They have 1 year, then the lizards will invade their planet and kill everyone. However, the eel-men have prepared for this...

Or have they?

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That is just the introduction to my setting. Pipperchip's suggestion, the Super Deadly Eel-Killing Plague of Swift Deadliness (Which is Deadly) has failed. As has all biological warfare. Hucking space rocks at the Eels won't work for various reasons, radiation won't work, and blocking sunlight is not an option.

The only option is a brutal ground war. Except it is not a ground war, because all the fighting will be underwater.

About 50-25% of the ship's the lizard men send will not make it to the surface, due to the Eel's rocketry defenses throughout the war. The cost of those ships, and the cost of anything isn't too relevant to the situation.

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Given all of these restrictions, How do the Lizard-men effectively conquer the watery planet and defeat the inhabitants? I'm not sure if modern guns could work underwater, what kind of fighting would happen down below?

Note - I am not aiming for xenocide, but the lizards do need to conquer more than ninety percent of the planet.

Answer 1

For most modern underwater combat, the primary means of delivering a deadly blow is to use large explosions and let the shockwaves moving through the water do the damage. For a rigid body like a ship or a submarine, this is very effective indeed, and it is also effective against things like human swimmers, since the shockwaves case damage as they pass through the differing areas of density inside the body (especially air filled spaces like the lungs).

It also works well on fully aquatic beings like fish, as is indicated by the use of dynamite, hand grenades and other explosive ordinance as improvised fishing gear. So the "easy" solution would be to drop explosive devices into the enemy ocean and kill them using shockwaves.

The difficulty comes when you look at the sheer volume that the enemy "eel people" have to hide in. You want to eliminate the enemy rapidly, without giving them time to regroup and launch a counter attack, and you need to ensure there are no gaps or voids in your coverage, so you need to create a 3D grid pattern of explosions under the oceans and water surfaces of the planet. Assuming you use a small nuclear depth charge with a kill radius of a kilometre, you will need at least 11 of these devices to reach from the surface of the ocean to the bottom of the Marianas Trench on Earth, and the Earth's oceans cover a surface area of 361,900,000 Km^2. Since the average depth of the Earth oceans is @3.6 Km, you will need to bombard the planet with @ 1302840000 nuclear devices to ensure 100% coverage for Earth. The alien planet has an even greater surface area so the number will be far greater (depending on the average depth of the oceans).

There is a possibility that effects like shockwaves reflecting off the various thermoclines or undersea terrain might cancel out or create voids in the coverage, so even the use of well over 100 million nuclear devices might not ensure you get everyone!

And this does not include costs like the delivery systems, mining and preparing the materials to make the devices, storage, guarding and transporting them to launch sites and so on. And unless you spend the extra money to ensure you have a "time on target" attack where you have instantaneous coverage (or at least a time interval measured in minutes or less), then there is also the possibility that the enemy can swim away from the wall of explosions. This solution will bankrupt the lizard civilization. Making much larger devices like the Tsar Bomba or even Gigaton range devices changes the cost curves somewhat, but need larger and possibly equally expensive delivery systems.

Somewhat less effective (but in terms of energy and resources also pretty high cost) would be to go into space and manoeuvre a "dinosaur killer" asteroid or comet into an orbit which impacts the enemy planet. There you have the additional issue that the impact will be so vast that there is a distinct possibility that large volumes of matter will be "splashed" onto your own planet (given the close proximity) and you will be battling the effects of global firestorms, toxic smog and dust clouds as incoming meteors reenter the atmosphere and are frictionally heated to thousands of degrees. Since we routinely find meteor debris from Mars and the Moon on Earth dating from the late heavy bombardment period, it is very certain that the debris from the enemy planet will hit you. (This problem might also arise if you are using vastly scaled up nuclear devices from solution one).

Even boiling the oceans using some sort of gigantic orbital microwave or mirror will require either too much time (they can deliver countermeasures or a counter attack), or you need to deploy improbably high energy levels to do so quickly. From the Atomic Rockets "Boom Table", it takes 6.6 × 10^26J to heat all the oceans of Terra to boiling. The Sun only puts out 3.9 × 10^26J each second (bolometric luminosity), so if the Lizard people can capture or generate that sort of energy they are already a KII level civilization and hardly need to worry about mere planetary conquest....

Answer 2

Targeting

You didn't say how much of the planet the Eel-Men were using, so I'll assume that they have densely populated cities and large uninhabited areas. There are lots of economics reasons why this would be expected. So, the first order of business is to figure out what needs to be destroyed, and what can/should be preserved. I presume the Eel-Men do not have enough resources to effectively patrol every cubic km of their ocean, and that defenses are concentrated around valuable targets. The interesting question is whether the Eel-Men leadership would put anti-orbital weapons on land to deny the use of land to the Lizard-Men, or if they would cede the land to concentrate on their precious underwater holdings.

Invasion

Land-based defenses would be susceptible to orbital bombardment, so I assume that the Eel-Men would not bother. In this case, the Lizard-Men should try to establish a beach-head literally on the beach, but approached from space, rather than the water, because in this world, the sea is more dangerous than the air. Of course, this already presents a huge problem for the Eel-Men, because logistics is now much, much easier for the Lizard Marines. Instead of dropping a huge infantry force and suffering massive casualties, they can just drop AA batteries over the centers of continents, which should be the least defended, and transport them to the edges of the continents, where they can interdict any missiles launched from underwater. This should make it much more effective to drop troops and supplies. In this manner, they can achieve air/space superiority over continents with a fairly low casualty rate.

Denial

The Lizard-Men can now use their orbital/air superiority to eliminate Eel-Man observation stations above water. Now the Eel-Men don't even know where the Lizard-Men are massing to attack and lack early warning until they approach the water. Of course, the Lizard-Men would use props and disinformation tactics to make it look like they were gathering near one juicy target while actually massing towards another one. This would force the Eel-Men to spread their forces thin to defend all vulnerable cities, or to commit a strong defense to the most likely target(s). The Lizard-Men get to shape the battle space to their advantage.

The one benefit the Eel-Men do have is that they can lightly defend the cities furthest towards the center of the ocean, and gather forces mostly around the continents (you didn't mention whether the Eel-Men liked all depths, or preferred to live on the continental shelves).

Attack

Finally, the Lizard-Men attack one or more high value targets (ideally the ones which would most demoralize the defending army to force an early surrender). They would use a diversionary force to draw Eel-Men to the wrong location, and then strike the actual targets if the Eel-Men take the bait.

The real risk here is that once the Lizard-Men go underwater, they are at an immediate disadvantage. Their entire war machine is likely optimized for land warfare, since that is how they evolved. They would need to spend years training underwater before launching the invasion strike so that they will be prepared. Even so, the further underwater they go, the more risk they assume. Operating at 30m depth would be difficult and risky, but usually survivable. If the Eel-Men can dive to, say, 1000m, and that is where their cities are, then the Lizard-Men must always operate in submarines once they attack, and the Eel-Men only need to compromise their hulls to defeat them.

Tactics

Obviously, torpedoes will be the most effective stand-off weapon, and the Lizard-Men would be well-advised to employ them liberally. Fielding big ships themselves would be needlessly expensive for the Eel-Men, as their best tactic would be swarming. Torpedoes are effective against large, slow-moving ships, but pretty useless against a swarm of 100 nearly-naked Eel-Men swimming at high speed and armed with attachable mines.

A torpedo requires a fairly large boat to carry it, so Lizard-Men will need decent-size submarines to bring the heavy firepower to the Eel-Man cities. Essentially, torpedoes are the only effective artillery the Lizard-Men can bring to bear on their underwater enemies, so large submarines are unavoidable. The threat of swarming swimmers means that they will also need point defense, which could be harpoon guns on turrets. However, this will become very expensive very quickly. Even if the harpoons are very small, the subs will not be able to carry an indefinite supply of them, and the Eel-Men can approach from literally every direction. Trying to defend an entire sphere of space at range is extremely difficult. If the effective range of the harpoons/fletchettes is only a dozen meters or so, then they will need many turrets all over the sub just to deal with swarmers. Of course, they can also eject depth charges to deal with un-armored swimmers, but again, there are only so many they can carry.

The Eel-Men can also bring ships to the fight, and they can quite effectively force the Lizard-men to fire their counter-measures with mass torpedoes. In fact, they don't even need to all be live. They can use mostly fake torpedoes to force defensive action, because the Lizard-Men will not be able to tell which are real and which are duds. Thus, the Eel-Men can clear the way for the swimmers, which then just swim up to the subs and plant charges, and swim away. They will take casualties, for sure, but they can make the fight very expensive for the Lizard-Men, who have to build the subs from scratch on land, while the Eel-Men have been building war machines for decades.

In fact, the Eel-Men may simply have leviathan ships which are massive torpedo boats which can overwhelm an entire squadron of Lizard subs' countermeasures. Surely the Eel-Men will have fought each other over sea floor and natural resources, so they should have this technology already. The ocean is a very inhospitable terrain for the Lizard-Men.

Although depth charges are quite damaging to unprotected swimmers, they are just as damaging if not more so to a sub operating near its maximum depth. If the Lizard-Men are required to dive 1000, 2000, 3000m to take key Eel-Man cities, then surely their subs will be vulnerable to even the slightest local overpressure caused by a detonating torpedo. This is like the effect where you can stand on an empty aluminum can, but if you just tap the side very lightly, it instantly collapses. Eel-Man torpedo barrages don't even need to hit the Lizard-Man subs. They just need to create enough shock wave near the subs to compromise their hull integrity. The weight of a 2000m water column will squish the subs like tin cans. This means that Lizard-Men will have to interdict all torpedoes at stand-off range, which seems nearly impossible. The lethal effective radius for an Eel-Man torpedo might be 100m or more, just because of overpressure deformation. Trying to keep all torpedoes outside a 100m sphere of a Lizard-Man sub might just be impossible at their level of technology.

Conclusion

I think the only ways the Lizard-Men can triumph are:

• The Eel-Men cannot naturally survive extreme depths and live on the continental shelf at no more than 100-200m depth.
• The Eel-Men have poor metallurgy skills and few or primitive war machines due to developing underwater (but this would make them less attractive for subjugation).
• The Lizard-Men have powerful industry and can drop raw metal on the planet from nearby asteroids and refine it quickly into war machines. Otherwise they have to mine it locally, which would be quite tedious.
• The Lizard-Men have more powerful computers, and can use acoustic counter-measures.

The last point is interesting, so I'll expand it a bit. Visual contact at depth will be difficult to impossible. If the Eel-Men live at depth, then they are probably capable of echolocation biologically, and may also be able to sense bioelectric fields for hunting prey. Torpedoes may also use sonar to locate their targets. On earth, subs try to hide, because there are few of them and the ocean is large. The Lizard-Men don't need to hide, because they are an invading army. They just need to disrupt enemy torpedoes. Thus, they can deploy acoustic drones which emit confounding sonar signals to muddy the battle-space. If Eel torpedoes can also get visual lock, then the Lizard subs and drones can also deploy ink counter-measures.

If Eel swimmers are acoustically sensitive due to their echolocation abilities, then Lizard-Man subs can just use intense sound as point defense against swimmers. The loud noise doesn't bother the Lizard-Men, but may actually be lethal to the delicate Eel-Men. And if they are bioelectric-sensitive, they could also use electrical shock near the sub to further repel Eel swimmers.

So, depending on the capabilities and technology level of each race, there are various weapons systems which can be deployed offensively and defensively. Choose their properties to decide what outcome you want!

Answer 3

Not having to kill off each and every member of the eel-men species makes the job somewhat simpler for the lizard-men. Based on the parameters set by the OP, my campaign plan is as follows

Phase 1. Reconnaissance

As the OP stated, the eel-men has effective A2/AD (Anti Access/Area Denial) system deployed. Enough to kill 25-50% of all incoming ships/units. Knowing this, the lizard-men would not deploy a lot of troops down in the first wave. Instead, the first phase of their operations would be to land recon units down to map out and gather intelligence about the location of as many A2/AD systems the eel-men has. A vast majority of these reconnaissance will be conducted by unmanned/remotely operated units, with some specialist teams deploying as C3 and to reconnoiter high-value targets like leadership and eel-men C3. These units will be deployed in low observable/stealth drop pods to avoid detection.

Phase 2. Penetration

The reconnaissance units on the ground will then serve to provide terminal guidance to lizard-men precision guided munitions, and follow on combat drones/UCAV/UCUV. This is the primary counter A2/AD phase of the campaign. The objective here is not so much as to destroy the systems outright, although they will destroy as much as possible, but to destroy enough systems (launchers, radar, C3) to achieve an acceptable casualty ratio on their main invasion force. The lizard-men need to ensure that, despite eel-men resistance, they will still be able to conduct effective military operations against eel-men targets. Some of these PGMs and drones will also target eel-men leadership in an effort to decapitate the enemy prior to the main assault.

Phase 3. Assault

Having effectively countered the enemy's A2/AD systems, and decapitated the enemy's leadership (to varying degrees of success), the lizard-men will deploy their assault units into the planet. These will be troop transports (with troops equipped to fight underwater for extended periods), cargo transports that carry field hospitals, field command posts, fire support systems, logistical systems, supplies, and so on, as well as combat vehicles (combat submersibles). Their objectives are to establish a beach head (or, sea head) where follow on forces can land without being attacked by the enemy and consolidate. I'll leave the make up of these forces to the OP, but I'm thinking they would deploy a multitude of unmanned systems that fight better underwater than terrestrial lizards do.

Phase 4. Follow on Operations

ASSUMING Phases 1-3 are reasonably successful, the lizard-men will then conduct operations aimed at finally defeating the eel-men's military and political structures. I'd imagine this would be militarily quite straightforward, as its a 'ground' campaign. They'd take ground, hold key locations, disrupt the enemy's supply lines, take and hold key production, power, and infrastructure facilities, etc. Pretty much what you'd expect in an invasion.

Phase 5. Occupation

ASSUMING Phase 4 was a success, then the lizard-men will start occupying the eel-men's planet. This is where a lot of interesting stories can be written. Resistance movements, secret police raids, work camps, forbidden love across species, etc. I don't think I need to elaborate much on this part as I'm sure the OP is itching to write his own take on this.

EDITED TO ADD:

As far as equipment go:

A lot of the fighting (at least by the lizard-men) will be with combat submersibles (manned or unmanned. A vast amount of their systems would be remotely operated). This would make the most sense since the lizards are terrestrial, and while they can operate underwater (like humans can operate underwater), they would not be able to match the eel-men's natural affinity for the medium. Submersibles can be made especially to fight under water.

Conventional firearms wouldn't work underwater because it requires oxygen to fire the charge. Also, bullets designed to fly in air wouldn't do much underwater. Instead, I see them using specially designed weapons that fire hydrodynamic flechette type rounds fired from cartridges with individual gas propellants (like CO2 cartridges, instead of powder).

FOLLOW ON EDIT:

When it comes to the question of WMDs, every army that ever contemplated on using them must weigh the gains of using them versus the effects. Using nuclear munitions underwater may have good effects on the target, but will also inevitably destroy the very resources the lizard-men wanted to begin with.

Also, any WMD with lingering effects, unless its something especially tailored to affect eel-men and nothing else, will affect the lizards too - which means that the lizards must operate wearing protective clothing, which will degrade the lizard-men's combat effectiveness over time.

Any WMD use will, therefore, be tactical in nature. A way to break deadlocks, destroy large formations, or destroy specific targets, timed to minimize lizard-men exposure, and dialled to minimize collateral damage.

Answer 4

There is no lack of information on the subject but Sonar kills.

Usage of High powered Sonars

Sound kills. Even more so underwater where it travels further. After a very quick search it would seem high powered sonars can cause damages to mammals up to hundreds of miles away from the point of origin. (Here I'm uncertain how depth comes in play). From there, all your Lizard Men need is an arsenal of Sonar weaponry (bombs, missiles, torpedoes, vehicle trolls). Starting from the few coast they can work in-water and setup a grid of sonar stations (to avoid the eel men moving around the oceans and avoiding the deadly 'traps')

The Lizard men do need to account for possible counter attacks involving artillery strikes unto their bases of operation and the such, but if they manage air superiority none of what the Eel men can do will matter much - it'll be game over and a simple task of clearing the oceans.

Here is just one of many websites that refer to documented events : http://www.scientificamerican.com/article/does-military-sonar-kill/

Answer 5

It's a little bit of a crude solution, but it would work: remember what happens when you pour salt on a snail? In the same way, you just need to find a way to cause a large chemical unbalance on their planet. If they live in freshwater, then adding enormous amounts of sodium would dehydrate and kill them quickly. Of course, the planet is big, and so are the oceans, but locally, within a mile of the salt canister touching down and dissolving, all life would be gone. If its a saltwater planet, use something else. If humans are good at one thing, it's polluting oceans. Finally, humanity will put this to (arguably) good use.