In WW2 combat mostly happened outside of cities ( with a few notable exceptions ), and as far as I know the same has been the case for every war before or since.
Why would it be different in the future? What changed?
Force disparity and lack of options to hide. Most Aurora conflicts will be similar to what happens when a Great Power bullies a smaller one in war; an isolated location (the planet) beset by vastly superior numbers of enemy combatants of peer or greater technological prowess. Frankly, the outcome of the battle isn't in doubt; the only question is the cost.
And modern day societies tend to be very squeamish when it comes to civilians caught in the crossfire. For defenders this is actually a
good thing; for an attacker not to come of as evil they have to be very careful in their engagements, while the defender has in theory at least a massive potential intelligence advantage in all those eyes and ears that could call in when the attacker is on the move.
In reality it's the other way around. The cities can't survive isolated but are dependent on supply/food from the countryside. The units controlling the countryside can seize supply/food and attack routes to deny it being delivered to cities.
That is an option if urban food production isn't a thing. However, that isn't really an option even when all food comes from more rural areas and the planetary population density is large enough; even outlying farms will be in the reach of city based rapid reaction forces, and getting to the next objective requires urban terrain anyway. And frankly? Cities are horrible to attack; too many angles and nearly every has a window to shoot from.
Real combat (especially of XIX-XXcc) was a mass warfare. They manned millions on the field, filled the whole lines of contact with their infantry units and artillery. Even now NATO have no such numbers of combatants and barrels - and in the Middle East we can see now this drift to city-centric main combat, while rural area is filled with guerrilla skirmishes, as I stated above. In Aurora we have even smaller units then NATO have now IRL, and they have to control the whole planet, not a separate problematic region.
The European tech advantage was so large that they managed to conquer and subjugate populations several times their own homeland's with tiny armies in comparison. The main reason you don't see that in the early 21st century is because the tech advantage is too expensive to exploit while the natives have acquired effective weaponry that's almost as good at hitting the most vulnerable sections of the conquering armies while the homefront is much more aware and much less willing to sacrifice lives on wars of conquest.
And, as it was stated above, there is no need in wide rural area in Aurora. You have vast energy resources with TN techs, so you can grow food in greenhouses and bacterial tanks, not in the field. You need area just to fill it with your suburban, to make your population happy on expanse.
Actually, energy production is a
major weakspot in TN civilizations. They require so much of it it's impossible to hide, and losing it would majorly impact a TN civilization's ability to supply its cities.
And if you have orbital observation force and meson cannons - than no enemy troop can survive in attack at your transport routes at this planet. Those troops can make kamikaze diversions (one troop - one diversion), but no regular warfare with area control, if they have no mesons. And if they have mesons and TN radar - they can attack any target on the planet or on the orbit at any time from any location. There is no line of contact, no rear area, and no masking outside of tech-dense urban and suburban with TN techs.
Yeah, if there's one gun type that needs to be incapable of operating in an atmosphere it's mesons. Otherwise all conquest will be 'nuke it until everything is dead' because the locals can resist too effectively.