Looking at Galifar in 894 YK, just before the start of the Last War, there is something interesting that should be noted whenever talk of which nations fought which nations, that is which nations bordered which nations. Certainly special operations can go deep behind enemy lines, or into nations that you don’t share a border with, but unless you’re engaged in an amphibious assault and support operation, which is difficult to maintain, nations fight other nations they border. If you want to have a detailed explanation of this, Winston Churchill’s history of World War 2 presents the difficulties that Stalin had in trying to convince the Allies to open a second front as soon as possible; even over the short distance between England and France, maintaining a front there would be difficult. An army without a stable supply line will not long survive, and while it is cheaper, by far, to ship goods by water, military campaigns are not easily maintained without good land access. The modern era, of course, is an exception to this, where goods can be brought in by air, and supplies are not bulky; an MRE (Meal Ready to Eat) is far more compact and long lasting than their medieval equivalents.
In Eberron, particularly if you consider my previous thoughts on the extensive use of magic, many of these problems would be alleviated, but not eliminated. Further, certainly an army can ‘live off the land’, similar to how Sherman cut through the South in the US Civil War, but such operations are limited in nature and are unsustainable over a long period of time. Had Sherman’s army been forced back along the same path they had taken, there would have been little resources for them to draw upon as they had already exhausted it. The Last War also lasted for a long period of time, and prolonged foraging is not something that an army of any significant size can maintain; the area that can be foraged does not double if the size of the army doubles.
Getting back to the issue of borders, if you look for pre Last War maps online, there are two that pop right up, one for 500 YK and one at 894 YK. Between those, you can see the borders shifted a little bit between the various nations; Thrane picked up the Harrowcrowns, a forest in north eastern Breland, and Karrnath gained the Blade desert south to the current southern border of Talenta Plains.
Comparing the 894 YK map to the 998 YK map, there are significant changes. There are of course the nations that cleaved off from the major nations, such as the Mror Holds and Zilagro, but more telling is the land that did and did not change hands between the major nations. Thrane lost its western half to Aundair; the city of Passage and the towns of Latheer, Bluevine, Ghalt, Marketplace, and Arcanix. It lost its south western chunk, the Black Caps, to Breland, which included the city of Xandrar. That said, it pushed its northern border up to the Aundair river, taking the town of Daskaran and the city of Thaliost. It also pushed its southern border down to the Brey river, taking the town of Olath and the city of Aruldusk. I found some differences between the borders of Breland and Cyre, but that could have just been a cartographical error; no cities or towns are within the disputed area.
This can be of campaign use in noting areas where there might be local resistance. Even when integrated into a larger nation for a lengthy period of time, people often develop deep nationalistic roots; consider the Irish and the English, or the Basque and the Spanish. Further, those Thranians outside of Thrane wouldn’t easily give up their worship of the Silver Flame, nor would followers of the Host just accept it, even if they were now in Thrane. Also, as a background element, styles would be different from one part of the nation to another, because styles in one nation weren’t the same as another, even if they were conquered. This might lead to interesting combinations; Host churches having architecture similar to Silver Flame cathedrals.
Next let us consider the distances involved in one nation invading another nation by passing through a third nation. By the end of the war, for Breland or Karrnath to fight one another, they would have had to travel through Cyre a distance of around 450 miles. Aundair and Cyre could fight, if they didn’t mind sailing through Scion Sound, past both Thaliost and Rekkenmark, a distance of around 350 miles. They could have gone overland through Thrane, but that’s a trek of 550 miles.
Supply trains moved at different rates, but generally speaking an army that outpaces its supply train doesn’t eat, and as they say, an army travels on its stomach. Taking real world numbers into consideration, as opposed to those in D&D, “the ox-cart could move a thousand pound load over 10 miles per day while a horse team could move the same load 32 miles per day at twice the speed on half the forage”; (oxen travel at about 2 miles an hour for 5 hours, horses travel about 4 miles an hour for 8 hours). Regarding the armies themselves, “Alexander’s army could routinely move at 13 miles a day, and separate cavalry units could cover 40 miles a day.” “On dry, unpaved roads a Roman legion (6,000 men) could move about 8 miles a day. In wet weather, movement was impossible at any speed. On paved roads, however, a legion could move 25-30 miles a day in all kinds of weather.”
Another thing to consider is that an army is not a point object, moving from place to place, but instead more of a wave, with a length. Consider the diagrams on this site, that show how a legion is broken up into scouts, a vanguard, surveyors, pioneers, legionaries (the bulk of the army), the baggage train, and the rearguard. This is the sort of travel one could expect through enemy territory so as to properly set up defenses each night. So having to travel up to 55 days through enemy territory to get to another enemy is impractical.
One interesting thing to note is that, except for weapon supplies, Warforged don’t need to set up camp or have a significant baggage train. They’ve been described as having stood in the rain for days without issue, they don’t need to sleep, and they have no need for food. Further, Warforged could transport non-Warforged in litters, so even if the commander of a Warforged unit wasn’t one as well, they could keep up with them. Certainly Warforged casters might 8 hours to regain their spells, but if they haven’t expended them, they could keep up with the rest of the group. While initially I can imagine Warforged being simply used to bolster the front line, it would make sense that they would be kept in reserve, so as to better respond to an unexpected enemy thrust (on a strategic level, not on the battlefield). Another use might be striking deep inside enemy territory, in places and in numbers that would not be expected. While small strike groups of ‘adventurers’ could teleport in and out, a unit of Warforged could march deep behind enemy lines and cause all sorts of trouble. This final idea may be the reason why Warforged are so despised; the only exposure to the Last War that regular folk might have would be the loss of someone they knew, or a unit of Warforged burning down their village.
Needless to say, unless they are allied with the nation they are travelling through, it is unlikely that one of the nations would grant freedom of movement of another’s armies through their territory. Even during a ceasefire, Breland, for instance, might think twice about allowing an Aundairian army large enough to threaten Cyre pass through its territory; if they were capable of extending their power that far, what would keep them from doing the same within Breland? On this principle, I think it would be unreasonable for Karrnath and Breland to have seriously engaged each other, and similarly with Cyre and Aundair. Their armies might have bumped into one another (which usually is a bad thing), while in a third nation’s territory, but the idea of Breland creating a beachhead in Karrnath without Cyre taking issue, is a challenging one to accept. Certainly, if Breland and Cyre were allied this might happen, but no nation remained allied with any other nation for long, so any beachhead would be temporary.
From this I would suggest that relations between these separated nations, post Last War, would be better than nations that shared a border. If texts suggest that, for instance, a Brelish soldier might have a base reaction of Unfriendly to any other nation’s soldiers, this would not apply to Karrnathian soldiers, as they rarely encountered one another.
All in all, when considering the Last War, it wouldn’t have been practical for it to have been an all out free for all brawl. The two pairs of separated nations would more likely than not have had somewhat cordial relations, and would have been more likely to ally against a nation they could squeeze between them; Brelish soldiers fighting to link up with Karrnathian soldiers in Cyre rings true to the fronts in World War 2. Plans might even have been drafted as to how the nation in between would be carved up, just as the Nazis and the Soviets carved up Poland.