You haven't made the connection between conducting raids on enemy territory and them automatically winning the battle on the main front. I'm not talking about using my entire fleet to do this, just two or three fleets of ships powerful enough to overwhelm unsupported space ports and fast enough to evade their main battle fleet
You're making a big assumption assuming a large fleet is slower than a small one. Fleet speed is independent of fleet size or strength, and this makes any payoff of said strategy much less significant and far riskier: if your raiding fleets get caught your opponent wins with much fewer losses than if there was a big fight. If such a tactical difference in fleet speed existed, it would, in fact, be
just the kind of possible incentive to not have large fleets that I'm talking about, as it would make raiding strategies more viable.
And I thought the connection was clear in the initial comparison: if someone destroys some of my infrastructure, but in the meantime I've effectively won the war, do I necessarily care about the infrastructure lost? This, of course, depends on what you consider to be acceptable losses for what you gain out of winning the war.
Sometimes concentration is legitimately the best strategy, and that's what you should do in those cases and you should not be arbitrarily penalized for doing so.
I'm putting this bolded line here to highlight that this is probably the central point of my argument - the rest of the stuff we're talking about is essentially details. Important details, but hardly the thesis of what I'm saying.
The issue is not that concentration should be penalized for its own sake - the issue is that not having any limits whatsoever on concentration means it's always the best strategy, and therefore an empire must always focus on having an objectively stronger military than its rivals or risk destruction, as opposed to having less of a focus on military but being able to fight tactically and come out ahead in defensive wars. If an empire can field a force capable of flooding your empire with raiding fleets strong enough to overcome whatever non-fleet defenses you have, to the point that it makes a significant difference in the outcome of the war, then they're probably capable of fielding a giant fleet that can beat yours and decisively win the war outright.
And related to this, an emphasis on concentration of force makes a single, aggressive, militaristic empire hit much harder, comparatively, than an alliance with equivalent totals of resource/production/tech capabilities. Non-aggressive play and attempts at getting non-conquest victory conditions seem to be at a disadvantage.
If you are significantly outclassed by a rival empire you obviously should not be expected to win without some legitimately brilliant maneuvers. But if you're somewhere in the ballpark in terms of strength, there should be some kind of equalizer that gives a smaller empire a fighting chance. Without some limit on how much force can be concentrated at one point, this seems to be not so likely to exist, which in turn incentivizes one and only one style of play - stronger fleets. No matter what.
I don't know where you've got this idea that you can simply ignore one front. Having a load of planets occupied and losing all your resource stations is not going to do wonders for your warscore, nor your ability to replace losses.
I don't know where you get the idea that "crush all resistance on one front and then turn your attention to the second front" means you'd be sitting around long enough to let things actually get occupied. Destroy one empire's fleet, then turn around and chase the others out/destroy them. Then start the occupation/mop-up process. The fewer losses you suffer by dealing with the fronts while they're split up plausibly balances the infrastructure you'd lose from raiding fleets before you deal with them.
This was exactly the behavior we saw in EU4 before the new fort system - kill everything and then carpet siege to completely knock one side out of the war permanently. Maybe you'd lose a province or two and/or get looted before you could deal with the other front, but it hardly mattered in the long run. I will grant that later in the game when you start getting techs for FTL-blockers and whatnot that it would more closely resemble the new EU4 fort system and tactical maneuvering would be more significant. But that also works both ways - FTL-blocking arguably hurts raiding tactics just as much as it would hurt a doomstack - if not more, due to a doomstack being more able to quickly destroy whatever's blocking its movement..
Combat doesn't work like that in Stellaris, ships can attack anything in range (which can be half way across the system with high end tech), they don't seem to be locked in either a combat or a non-combat state.
There's obviously some combat vs. non-combat state, even if it's just at the level of "currently firing lasers".
An obvious (yet naive) approach is to think of all ships in a system as contributing to ships involved in the battle - a bit sloppy, but loose abstractions are sometimes a necessity and this is not the sole possible solution. But it absolutely is sufficient for proof-of-concept. Actual implementation is definitely a concern for further refinement, though.
As I have stated elsewhere, I'm more than open to entirely different incentives that aren't explicitly related to size of a combat - as long as they're something meaningful. I'm just not convinced what we've seen so far shows that those exist.
It's probably worth keeping in mind that the reason I started replying to this thread was in response to someone who was implying that giant stacks are a good thing and any incentives to not have them are bad - a position I don't think you agree with, seeing as your argument so far is that there are sufficient incentives for fleet splitting due to things like mobility and that there are no need for additional incentives against giant fleets.
They've fought a very brief war with a one system empire that had just achieved FTL and another war with the blue lizard warrior people.
And neither of those are at all a demonstration of what happens in a multi-front war against a comparatively-strong alliance. And with the disadvantages the AI has been put through due to the nature of the Blorg stream, I'm not sure even the upcoming war would be a fair representation of how things would work out.
I thought about this some more. It depends a lot on what type of FTL you're using, and what type of FTL your opponent has.
Wormholes almost necessitate splitting your fleet. You have to defend those stations, or else you're quickly going to find yourself unable to even move. Simultaneously, instantaneous FTL allows separate fleets to mutually support one another: you can keep your forces split up, but they can instantaneously converge if attacked en mass, provided your advance doesn't outpace the construction of your wormhole stations.
If you're using hyperlanes, you decision to consolidate or spread out is dictated by the numbers of the enemy and the geography of the hyperlanes. If you have one choke point, you have to concentrate. If you have more than one choke point but are outnumbered, you have to sacrifice ground and still bunch up. If you're not outnumbered, you can decide how you want to deploy depending on the layout.
If you're using warp, you have to play it by ear a bit. You have flexibility, but not speed so you need to figure out how to negate the enemies mobility advantage and exploit their restricted ability to move.
Permutations of these basic principles occur depending on whether you're attacking or defending, the layout of the map, the tech levels involved, and how much knowledge you have of the enemy.
I'll agree that mobility is a concern with regards to how you split up your fleet, I'm just not sure it's enough. Again, the behavior we keep seeing again and again with Paradox games is that concentration of force is always better unless there are things that explicitly prevent that force from being projected at will. If mobility concerns actually do affect things enough to require multiple fleets for multiple fronts, then that's absolutely fine and there's no reason for additional incentives. But I'm not convinced that's the case, and the issue is more along the lines of what overall gameplay this incentivizes (stronger fleets all the time regardless of ethos) rather than explicit tactical incentives.
Throw in the fact that Defense Stations work multiplicatively - not additively - on the fleets they support and that doomstack's gonna have a terribly rough time.
Was this actually said somewhere? If it was I must have missed it. And if anything, that just makes fleet splitting even less attractive for an aggressor - you need to be able to punch through a defense station with overwhelming force in order to make any gains. It would alleviate my concerns of every empire being forced to build as strong a military as possible, though.
