Rome was constantly at war. Here is a list of battles. Check out the time period from 343 BC to the establishment of the Empire in 30 BC. A 300 year time period with 49 separately listed wars. Rome was also rich. Doesn't need much explanation, but by 30 BC Rome had acquired the wealth for Augustus to turn Rome into a city of marble.

Rome became rich through conquest. That 300 year period corresponded with rome expanding from a single city to the entire Mediterranean Basin (and norther Gaul to boot). You need your country to be continually expanding. But Rome also had at least one permanent enemy in Persia; a land more or less just as rich and able to fight wars off and on with Rome over time.

You can have several rich, prosperous countries locked in battle with each other, as long as they all have opportunities to expand into other parts of the world, growing rich off of decades or centuries of conquest in the process.

In theory yes

But only if this is an Orwellian war rather than a real one.

For those of you who only pretend to have read Orwell's 1984, the world is kept in a constant state of war between three powers. Which power you're currently at war with or allied to is not important, what's important is to always be at war. It gives you a vent to dispose of violent or rebellious elements of society, it gives a rite of passage for young men, it allows you to always point to something to say "you must toe the line or they win".

To maintain a wealthy society you can't have a large proportion of your economy tied to the war unless you're winning. To keep the war running for long periods on a static front you can't be winning.

And so the war becomes Orwellian, maintained from above by agreement between the leaders of the powers as a benefit to all of them, but regulated in such a way that it doesn't do significant damage to the economy of any participant.

I think the easiest solve for this, if it’s possible in your world, is that in addition to fighting the unending war, a portion of the powerful countries’ military is employed elsewhere and engaged it maintaining imperial colonies - that is, the superpowers engaged in fighting off the invasion are drawing wealth (and probably conscripts too) from other parts of the world that they have subjected politically, militarily, and economically. This is basically how it has worked/works in real history.

As far as other “questionable government practices”, I would think that at the bare minimum there would be some internal political suppression (ie of an anti-war movement, etc - people are bound to blame their leaders for such a long war), and with all of the above I’d also assume that there’d be some internal disparity of wealth/class.

In most wars one side gets a small advantage, uses that to get a slightly larger advantage and snowballs into winning the whole thing.

To avoid that, you need a strong home field advantage.

Let one side have a strong cavalry, unbeatable in open areas.

Let the other have more people and better infantry, unbeatable in uneven mountainous terrain.

Or let there be some other reason the front lines get locked in place.

So, the war has been going for a few years, and there is no end in sight. People start asking about the purpose of it all.

Attacking and conquering places seems pointless, since the enemy will only take them back later.

Raiding makes some sense, that is attacking, looting and retreating. However, areas near the front line will get looted out and raiding too will slow down.

Defending makes perfect sense. You don't want your family or yourself to die after all. Have to stop those enemy raiders.

So, you get a defensive war. City walls, fortifications, maybe a full Chinese/Hadrian-style Wall.

At this point there will not be much fighting, so the war can go on without ruining the economy. Basically forever.

It depends...

A war that does not threaten the core lands, as we see with the current wars waged by the USA, for example, can be run continuously with not much ill effect on the economy.

Likewise, a war of expansion, as in the Roman Empire example already mentioned, can finance itself by plunder offsetting any economic downside.

Essentially, as long as the core lands are not under threat, a country can decide how much resources it wants to dedicate to the war.

However, once the war turns against the country, and it becomes a matter of survival, choice goes out the window. Now, the country needs to bring resources to the fight even if it means damaging the economy. At the end of this escalation, the entire economy is dedicated to the war.

Stop thinking about World War I. With technology equal to that of Ancient Rome, you're not going to get near the logistical capability to support that many people in that small of a space for any length of time.

What you want is something like China's Great Wall, or Hadrian's Wall in Britain. Those are actually both very topical for you, as they are both fortifications of some size which were maintained by very prosperous civilizations. Look also at Rome's Germanic frontier and how that was maintained.

In addition, if this is truly a multinational fight, with foreign reinforcements coming in, your country would probably have established routes of transit heading to 'the wall' with many businesses, small and large, designed to extract money and resources from those military men in various fulfilling ways. (Think brothels, inns, tourist traps, bars, et cetera.)

If you want to make this especially attractive, embed some kind of economic incentive in this 'not quite human' enemy. Maybe each of the enemy has a flawless diamond in its forehead for a third eye. Maybe their pancreases contain a rare substance needed for religious rituals. Maybe they all carry around nuggets of pure gold. This would help with the motivation for people to put up a lot of expense to equip themselves and travel many miles to help fight.

The source of "wealth" in ancient times was effectively agricultural crops, natural resources (mines for precious metals, marbles, jewels). Cotton, linen (from the flax plant), hemp, and various spices are all agricultural; technically silk is an animal product, but treated much like an agricultural product.

Give all your countries arable land, and limit trade severely. Make them expert farmers/ranchers. They can be self-sufficient and rich, they are turning dirt into wealth by growing food.

Better sanitation results in fewer deaths and thus more people for labor on farms, mines, and other productions, and the excess people can be expended in the war efforts. This can be a conscious decision of the despots in charge; to keep only as many people as it takes to provide the labor the country needs, and "cull their herd" by keeping the best (people with skills or beauty, artists and story tellers, dancers and singers) and sending others to the front to fight for more space. Let people find glory in fighting, rising in the ranks. For skilled fighters, the military can be a route to marriage and wealth as a plantation administrator (baron or lord or some title like that).

Natural resources can feed, clothe, and provide luxury for many centuries. Labor turns dirt and water into foodstuffs, even in early medieval times with no machinery, one man could do the labor to feed four people indefinitely. It would be about 16 hours of work a day, but not hard labor all day. A family of four could get by with 4-5 hours of work for each, each day. Both frontier families in early America and modern survivalists prove it, and hunting is not a necessity (although there is no reason your characters cannot hunt or ranch).

That disparity in the work required also leaves many people to provide secondary "derived" services. Making products out of the agricultural and mined products. Cloth, wood stuffs like furniture and boards, forged and refined metal products, paints, sculpture, flours, baked goods, etc.

Along with a large excess of people, thus the wars provide population control, and some small opportunity for the royal families to grab more territory, or a valuable natural resource. The most natural cause of such endless war would be over the natural resources themselves. Streams, ponds or lakes, mines, farmland, woods or hunting ground, fishing areas.

Israel has been in a state of war andor pseudo-war since it's formation yet is considered to be an economy stable and powerful country where the quality of life (and salaries) are quite high.

The question isn't if there is a constant war, the question is how good are they in holding the front line in place andor advancing them, if the front line reaches cities it would be impossible to have a strong economy as that requires investments and no one wants to invest in a place that's about to be taken over by an enemy, but if people are sure that the front line will either stay in place or will get further away I see no problems with it.

I suppose

Take into account George Orwell's 1984. The entire country of Oceania is at constant war. I do not know if it would be considered a strong economy by today's standards, but it wasn't completely unoperational.

War constantly drains resources of a country, but giving the labor force a motive to work harder. Factories can now operate at max potential and flood the military with war goods while paying out lots of money to the general public.

A war is fairly good for an economy, as WWII brought America completely out of the Great Depression. A country at constant war would constantly benefit from the economical boost provided by a war, so in theory it could have a strong economy.

Ancient Rome level technology cannot maintain continent-length high-density battlefronts.

The powers in WW1 couldn't do it. Every one of the powers -- France, Germany, Russia and the UK -- was destroying itself trying to maintain the battlefronts.

France and especially the UK was a bit better off; but it was mortgaging its Empire (to the USA) in order to stave off the decline.

The German strategy was to try to get the other foes to burn themselves out before they burned out. It worked on Russia and was close to working on France and possibly the UK when their unrestricted submarine warfare brought the USA into the war.

These where industrial civilizations with insanely cheaper transportation and more effective logistics, insanely higher infrastructure, insanely more productive agriculture, stronger central states, and population densities than ancient Rome. Collectively they had a huge percentage of the world's resources at their beck and call. And even after the fall of Russia and the ending of the Eastern front, the relatively short Western front was destroying their civilizations.

The War would have to be far colder than WW1.

Roman armies didn't do anything approaching the continent-wide battle fronts you are talking about. They where impressive in their ability to project force over continent-scale distances, but they could not maintain battlefields the length of continents. These two tasks are at least an order of magnitude separate.

Even most 19th century warfare looked more like Roman warfare than WW1.

Any 4x strategy game will be a decent simulation of how it could work. Start a game on largest possible map with a lot of opposing factions. Reach a desired tech level then start wars with everyone but don't conquer anyone; just constantly do battles with them. See whether you will be able to stabilize your civilization in such situation. I guess you would not be able to do that.

That said, your question basically boils down to "whether my society could prosper if I make daily sacrifices of manpower and resources to huge trash dump". If your income of manpower and resources is larger than the spending, you will prosper.

As was explained in the other answer, biggest limit is the people. Nowadays in Information age population increase tops at 3.8%. Let's say your country have 1kk people in it (HUGE population for High Fantasy setting). It means that it can realistically give you 38000 new soldiers per year IF ABSOLUTELY EVERYONE is conscripted and you have zero death rate before conscription age. This is pretty harsh conditions for a constant war, in my opinion, I hope your specific worldbuilding have enough handwaving for that.