From my perspective, there's no such thing as a 'normal' war. Given the relative value of each side to each other, I can't see any material reason for them to go to war and expend so many resources on harming each other. Are you talking about combat as a negotiation tactic for trade?

Who are they communicating with? If it is just among them, possession of the device has not intrinsic value: if any of the sides decides to close communication the device becomes worthless. So deciding this with a war is kind of pointless; the winner gets just a piece of junk. And Tim B II makes a great point, without FTL the times involved makes anything far from "normal" ("President, if you do not send a fleet right now our enemies could win a century from now")

You want logistics and strategy, not tactics. Tactics, generally, is the decision made once the battle start, the forward thinking is strategic and logistic decisions.

Also, why are they having a war? It's 80 years there, 80 years back. That a minimum of 160 years for the return of spoils.

It's my understanding that war is nothing more than a dispute about what peace should be like.

@candied_orange I agree. I added details based on comments

@Halfthawed - Agree, there is no near term spoils. They are warring over long- term planetary crises which only the other can avert. Think if an alien culture could reverse our global warming crisis before collapse many decades from now.

@Halfthawed sort of, maybe I’m thinking more about diplomacy than tactics, the communication exchange (which is also a resource - exchange of technology and culture) is the only real-time weapon they have

Are we assuming the "quantum device" is a magical plot device rather than an actual device based on entanglement as we know it today? Entanglement can't actually be used like that according to current known physics.

" long- term planetary crises which only the other can avert" ... then war is completely the wrong answer. Even if you win, now what? How do you coerce the loser into helping?

@Móż, classical blackmail? Help us and we will not nuke all you major and minor cities? (Thats would lead to "revenge war" - a good plot for continuation). For my opinion better reason would be good old xenophpbia. Some "Emperor of humanity" seizes power and now we will not let xenos to live!

Brief question about the "quantum device" - why is it 6ly away from each planet? from what I know how quantum entanglement works, you need 2 devices that have been quantum-synched - one at the Planet A, one at Planet B, kind of like telephones. It really does not help you to have your telephone 6 lightyears away. Or is that kind of like a... quantum repeater? Planet A quantum-phones to Quantum Device, Quantum Device reroutes to Planet B's quantum-phone? But why would you need the Quantum Device then - entanglement doesn't care about distance...

@ksbes but each side has something the other side needs... it's more like "do what I want or I'll kill myself". The other side has ~160 years to come up with a solution because that's how deep the transfer pipeline is. Ie, there's 160 years of shipments in transit, so if you stop now they have that long to fix the problem.

Larry Niven in his "Known Space" series has done quite a lot on non-FTL interstellar warfare. The primary requirements are some kind of drive that gets relativistic speeds, and warriors that live long enough to get from one star system to another.

If you want hard SF you have to drop the idea that quantum entanglement/quantum teleportation can be used for FTL communication, that's impossible according to the fundamental equations of quantum physics, see here. On quantum teleportation specifically (since you linked to an article on it), it requires a "classical channel" where one experimenter sends the other a message about what variables to measure via conventional signals, see here.

@Hypnosifl - No I'm sorry, the way to get hard SF responses is through the use of the [hard-science] tag. Other stories are welcome to invent "warp cores" or "hyperspace" or "cryogenic hibernation" indiscriminately to fit the plot.

I don't think there's any rule that comments about whether something would fit a hard SF setting are only relevant if someone puts that particular tag in their question (certainly not everyone who comes to asks questions will be aware of that convention), and you did mention you were trying to create "the most realistic hard sci-fi human/alien conflict scenario" in your question. You might consider modifying the question a bit to say something to the effect that you want a scenario where communication can be FTL but movement of massive objects like ships or missiles must be STL.

Just more handwavium than what can work. Read that article 200 years from now, many physicists are getting paychecks to prove your "Quantum impossibility" comment wrong. This is standard sci-fi "move current theories 200 years ahead" stuff.