Hm, well, let's see. And forgive me for a bit of incomplete knowledge, as I mainly deal with the rest of the multiverse instead of the core canon. If you want to talk about those dudes I will drone on forever.
Also note that everything below is covered more extensively and with (somewhat) less jokes on universe. The rest is purely conjecture/speculation and is not necessarily canon based on my running a lot of large scale battles in D&D and owning an eleven year old copy of Landmark Thucydides. So, you know, kind of an expert on war over here. Ha ha. Anyway.
Lets talk about military logistics. **Noxus **is essentially a huge melting pot of other cultures and peoples, and as a result has a huge, multicultural army trained in the most effective tactics and equipped with the deadliest weapons from defeated or absorbed nations. Because Noxus is (ostensibly) a meritocracy, this army is led by the baddest dudes around -- but individual soldiers can think and act freely if their commander gets taken out, so they're capable of performing well even without any active leadership. There's also the subject of Singed, who spends most of his time mixing biochemical weapons that he likes to "deploy on accident" for "super awesome tests." Collectively they're very hierarchical and organized, but individually they're unpredictable and patriotic. A scary combination.
The boys in Demacia are quite a bit different. They're an anti-magic monarchy and a kind of rigid, structured nanny state, cloistered off from the rest of the world but martially proficient in the event they need to attack or defend. Historically very powerful, they're also a bit fanatical in nature -- almost like an army of crusader-knights. Then there's Galio, who literally comes to life just to smoosh things and absorb enemy spells, which helps him smoosh more things. Any invading army would have their magic dampened and then have to fight on Demacian home turf against crazy strong soldiers. Not sure how the Demacians would do offensively. They don't really get out much.
The Freljord is a series of wild cards wrapped in another, larger wild card. The Freljord itself, much like Siberia in WW2, is its first line of defense. The people of the Freljord know how to deal with sub zero temperatures, unforgiving blizzards, and a bleak-ass tundra sparsely populated by murder vikings and (like in WW2) sixty-foot-tall snow monsters. After that it's down to who you're fighting. Ashe has a gigantic army of basically everyone, if I remember correctly. No matter how bad your dudes are, the Avarosans can overwhelm opponents with sheer numbers. The Winter's Claw is much smaller, but every single one of them is a hardened survivor capable of killing a dozen or more men in under a minute. Each of these factions of course is led by their respective champions, whose abilities also need to be taken into account.
Finally there's the Frostguard. Nobody really knows their true nature yet, and I guarantee Lissandra would see any attack coming months in advance. Were an invading army to get past the Winter's Claw and the Avarosans (Liss is a master manipulator, remember. Also I main her so I'm hella biased), and begin to break through the Frostguard citadel, there's at least a small chance (I actually don't know if she can just call them up like that. Gonna go with probably not) you're instantly dealing with a nigh-unkillable eldritch ice horror from an age long past. If she can just summon one up. Which she very likely can't.
Shurima kind of has the same thing going for it. It's a massive desert with nothing in it but sand and an angry talking crocodile and sometimes Rek'Sai, so any marching army immediately has to overcome all those hazards in addition to the unforgiving geography (and sometimes Rek'Sai). You've got two 'emperors' (I guess one is real, whatever) raising giant armies in their neato sandy castles, but both happen to deeply hate one another -- making it unlikely they would ally. Still, they're both ancient magical ascended (in Xerath's case, sort of ascended. Kind of. If you think about it), and they have very large and martially proficient armies trained to fight on sweltering sand dunes in the middle of the afternoon. They also probably recruit mages, or at least a handful of magical-type-dudes, so you're dealing with magic as well. And god forbid you knock the sun disk during your definitely-well-thought-out-siege. Seeing as how messing with that thing went so well the first time (spoiler: I did not go so well the first time). Plus if you accidentally march to Icathia because your maps are bad... well. Well.
Yeah, good luck invading Shurima. I hope you brought water and also your dumb idiot brain, you big dumb idiot.
I think **Piltover **and Zaun would be attacked before they fought anybody. That's pure conjecture, though; I'm not sure if they even have a standing military. I do think you're looking at a Zaunite-led guerrilla resistance consisting of jacked robot people, human-animal-chemical hybrids, time kids, engineered plagues, and whatever other nightmare Technomancer shit Zaun's bohemian weirdos decide might kick the invaders out. It might be easy to conquer, but it would be impossible to hold considering how advanced and deadly and difficult to track their weapons are. In my opinion. Again, **pure **conjecture, as is most of this stuff.
When it comes to Ionia, I'll let that one sit for now. Insert plug for universe here (seriously, though, the work on there is really really good). Hope this fires your imagination!
