!!!!!!!Effort post incoming!!!!!!!
We've seen hygp speculate on why people wage war, but there's another aspect we're still in the dark about. I'd like to speculate on HOW people wage war.
The first thing I feel like needs to be addressed is how the fuck are borders decided? Historically in the real world, it reflects how much land your military can cover/patrol. This is still somewhat the case in MfD as we saw Rock initiate war by killing our border patrols. However, the two situations aren't really analogous. You can't just march your battalions all over enemy land in secret - and if you could, that would be your land, not your enemy's. Compare that to ninja warfare where a single jonin can be anywhere at any time and obliterate swaths of chuunin and genin a day at a time. Sprinting across the entirety of the EN doesn't even take two weeks, compared to the long slog that was moving ground troops in the middle ages. So how would you define borders in this context? My current model is that different countries probably have different opinions about where the borders are, but they approximately represent a statement from the local village: "Within our borders, we ALWAYS have ninja close enough to kill you at any given moment." There's only so many ninja to patrol your claimed territory, so you can only claim you own as much land as you can immediately repel attackers from. You could have your jonin wandering around and murdering people in other countries, and indeed they likely do sometimes, but that leaves you open to the enemy doing the same with their jonin, and then your statement claiming you can immediately repel them isn't true - it's not really "your" territory at that point, it's contested, and if they get a foothold it may become theirs entirely.
So fundamentally, the ownership of land comes down to your ability to have ninja presence in all of your claimed land, or at least, to be present enough that the enemy can't be sure if you're there or not. The borders are like electron shells, where there's high probability of finding ninja anywhere, and the enemy has to chance an encounter with defenders to go anywhere. However - it's not as simple as just having ninja around. Ninja are too difficult to pin down to engage in effective invasions. Rather, you attack the most important aspect of all warfare - logistics. Ninja that are playing defense and occupying your territory need to eat, resupply, and receive medical treatment, and a vast majority of the borders are multiple days away from the village. That means either local civilian villages are critical to providing those supplies, or you need ninja runners, likely genin or chuunin, sprinting all over to deliver food and supplies in storage seals. An enemy village waging war on another nation would then attack these supply chains while expanding their own. If your enemy can't feed themselves, they have to retreat to a place they can resupply from. Conversely, if you can provide for yourself further and further from home, you are capable of having patrolling ninja further and further away from your village, which is ultimately what border expansion is.
We saw some of this strategy in the Hashirama ?interlude? where the Senju and Uchiha were starving each other to death by constantly destroying food supplies and farms. Modern warfare applies this tactic too, but with a twist. Larger armies means you can afford to actually defend these locations, just not all of them. It's usually better and easier for both sides to simply kill each other directly and leave the natural resources intact, those only become targets of last resort when you don't know where the enemy is. This turns warfare into a Spy vs Spy type situation, which is appropriately a core aspect of being a ninja. Rather than dedicating forces to hard targets, you can trick your enemy into believing your forces are elsewhere and ambush their undefended regions. Or you can trick them into thinking one area's forces are weaker than they actually are, a la USOUD. However, the enemy would know these tricks too and be on the lookout to pull the exact same thing. Thus, each side is intentionally leaking Intel, trying to deceive the other into over-dedicating their forces to an empty location or ambushing with a much larger force and wiping them out. The victors are decided not just by who has the larger army but who has the better intelligence gathering, and who is better at pulling the wool over the eyes of their enemy.
What would a typical operation look like, then? Rock Genin comes to Funkytown to find out what he can. Rocky is a disposable grunt, and only expected to determine if the village is relevant to the local ninja, he's not experienced enough to know how to blend in, or notice a disguised ninja. A Leaf chuunin hiding amongst the farmers notices the intruder and surveils him to determine if he can be used against Rock or not. Ultimately, Rocky leaves before Leafy decides to assassinate him, as he found out a caravan of steel would be coming through. Leafy would find out from the farmers that Rocky will return with more forces to intercept the caravan, and reports this to a nearby runner, likely another chuunin or a genin, who would relay plans from Leaf. From there it becomes an escalating game of who is better prepared to take on the other. Rocky failed to notice Leafy's presence, likely giving Leaf village the edge. However, were Rock to instead determine the region deserves more direct focus, and send an infiltrator similar to Mari, Leafy may not notice the new foe. The infiltrator might even trick Leafy into providing bad Intel or bringing a force of Leaf nin that end up getting wiped out.
Ultimately in the scenario provided, a steel caravan is the centerpiece of the struggle, but the conflict would grow beyond it to become who is sending the correct number of ninja for a showdown. If Rock overcommits, Leaf could kill their thinned ranks elsewhere. If they undercommit, Leaf would successfully fend them off. Reversing the roles would demonstrate similar consequences for Leaf, either they lose the territory or get invaded elsewhere, and more importantly, lose a chunk of their military. And these types of conflict are happening all over the place, all at once. This is why Thinker clans and strategists are so crucial.
So, why haven't the Goketsu been sent on any of these types of missions yet? Well, think about their specialties. Mari is a jonin infiltrator, if Leafy hasn't noticed anything important enough to warrant her attention yet it would be a waste to send one of Leaf's few jonin. The rest of the clan are combat specialists. The War just broke out, and both sides are mostly gathering information and killing scouts they come across. Major skirmishes haven't actually been planned yet, though I'm sure they're coming together, and when they do that's when I expect Akane, Yuno, Haru, and Kei will be sent to assassinate/murder some chuunin or back up a jonin against another jonin. Mari is pretty good at everything so there's a lot they could use her for. I doubt Noburi will be deployed because he's so powerful on defense and Hazou is more useful as a sealmaster but I can see him getting sent when he heals. The crippled Goketsu are already being deployed - they're the messenger/supply runner role I described above.
To put it in terms of chess, we are only in the opening, and prepping for the midgame. We've claimed some early material in the Battle of Five Clans, but ultimately everyone is still moving their pawns (genin) around to establish intelligence, letting the minor pieces (chuunin) control space. Once the board opens up, the rooks (jonin) will start sweeping material away. And Queens (essies) can do whatever they want, wherever they are needed. There's no king in this analogy, whoever loses more material loses the war. And right now, Rock and Cloud are both up in material. There's no checkmates to go for, we have to beat them in tactics.
