The current longest standing treaty is the Anglo-Portuguese Alliance which dates from 1386, so history says yes, it is possible for countries to remain at peace for over 500 years.

Of course Britain and Portugal aren't rubbing shoulders day to day along a common border, but for much of that period were rubbing shoulders on imperial expansion. Something that led to major conflicts between European powers for the most part, but that treaty held.

A union or federation of countries

You might want to go with a federation of some kind. In this example each kingdom/barony is its own self managed kingdom, but they collectively meet to decide on the legislation that will affect all countries/states. In this way you could have an entity like the EU, or United States. A super country formed from many smaller countries.

The idea of a federation is somewhat flexible, so you could increase or decrease centralized power as needed. For example, no centralized military.

The solidarity of these different countries working together over 500 years comes from the fact that they treat themselves as a single larger entity, and then derive benefits from it. For example economic benefit (free trade with each other, bail outs), military benefit (attack any country and you are attacking the collective), and so on. In your case there is an inbuilt benefit of "us vs them".

Us vs The Army of Darkness.

A super entity like this would be about as stable as a country. So its possible that it could last 500 years. Although like in all scenarios, political and technological change can cause a lot of upheaval.

Edit:

To add some excellent examples of lasting allegiances from the comments:

… a historical example that lasted the sort of timescale the OP
is asking about: the Holy Roman Empire. (Unfortunately one or two
little disagreements means that describing the constituent kingdoms as
"at peace" for that time would be a bit of a stretch.)

by @Martin Bonner,

More obscurely, the political alliance of city states in the Indus
River Valley appear to have had at least a 500 year period of peace
given the archeological record, although the peace didn't last forever

by @ohwilleke.

Yes, if some of these conditions were met.

1. Any sustained threat, real or imagined, to all five.


2. No shortage of resources


3. Mutually beneficial trade arrangements


4. Highly defensible natural barriers.


5. Industries dependent on the landscapes


6. Intermarriage and families living in multiple kingdoms

In other words, strong familial and economic ties and dependencies tend to make the costs of war high and the means to acquisition of resources far easier through trade. Common cultures and familial ties puts societal pressures on peace vs war as well. Add to that a common threat, even if it's imagined, will keep them united against that threat.

Yes, if they are island nations.

Invasion across a stretch of water is somewhere between ruinously expensive and horribly risky, up to impossible. As I seem to keep pointing out in all sort of contexts, the United Kingdom has not been invaded since 1066, and I believe that since the invention of gunpowder the only way to invade an island with strong coastal defences is to starve it into submission. Which itself is hard, if it is largely self-sufficient. Naval blockades are normally quite surmountable in any case.

So the question is, what can the armies of darkness do, that ordinary human armies and navies cannot do? One answer might be, swim! (Swim like fish, 20 or 200 miles, without being sunk by weight of weaponry. Evil mer-people? ) Or fly, on dragons, before the invention of aircraft. Magic would provide any other answer you care to dream up.

I haven't researched the history of Indonesia, which might confirm or shoot down my views. I think that's the only archipelago formed of many large islands on Earth. The British Isles have only two large islands.

Scavengers

All the countries have very similar power. A war between them has very low chances of a meaningful victory, it will be more death than reward since the armies are very similar in strength. But the most important thing is the neighbours: each kingdom is looking for an opportunity to devour another kingdom.

So, since the armies are very similar, if two kingdoms start a war it would take a lot of time to one kingdom erode the other kingdom -without losing much strength compared to it- enough to conquer it. It's very likely that during the process, another neighbouring kingdom will take advantage of the opportunity and conquer the weakest one, maybe even both.

That is why any kingdom has started a war yet with another kingdom, they are afraid of what could do those "neutral" neighbours.

Each one has something of importance from another

Imagine that one kingdom provides food to all of them, while other provides ores and metals to the others, another provides wood and furniture, other provides tools, etc. Each one needs the other 4 to work properly. During the centuries no kingdom was able to become self-sufficient, so all of them needs to trade each month with the others two. That is why no one can make war with another, or they will lose something important.

Remember that this doesn't need to be only trade and resources. It can be a powerful shared religion, so the kingdoms are full of sacred places and buildings which can't be profaned with war, or (weirdly) each kingdom has sacred and with a lot of cultural importance relic of another kingdom, so if a war begins the relics will be destroyed. Or simply, there are a lot of political marriages, the royal families are very joined by political, holy and sanguine bonds.

A greater evil

All the kingdoms of the region of Itheria are very weak compared to the great kingdom of Airehti (or whatever, maybe even it isn't a kingdom, just a powerful necromancer... like Ainz Ooal Gown). Only forming a union between them they are able to compare strength with that kingdom and face it. Any war will become the whole region vulnerable to external threats. So the only thing they need is a charismatic king who make a pact with all the kingdoms.

Mutually Assured Destruction

To add to the other answers, consider what would happen if one kingdom successfully took over another. Assuming all the kingdoms are practically equal in size, strength, and defensiveness, we now have three kingdoms of power X and one kingdom of power 2X. The newly powerful kingdom is now a threat to the other three, because it has the power to invade another kingdom and if it does it becomes (theoretically) unstoppable.

So, either through explicit treaties or an unspoken agreement, the 5 kingdoms remain at essentially a stalemate for 500 years. And if you're stuck in a stalemate, my not make it peaceful and get trade benefits out of it.

(I know there's an old story along these lines, but for 3 countries; I can't remember enough to find it)

It depends. Many kings weren't conquerors, because conquest is an extremely costly act. A reason for war was either:

• Resource shortage, e.g. hunger, need for ore


• Control, e.g. having control over trade routes, or maybe have access to the ocean for trade


• Religion


• "Race"


• Rivalry


• Ambitious plans for expansions, but as said, not many kings had those ambitions

Now if none of these factors are present in your kingdoms, enduring peace is very likely, as people need a motivation for war.
So yes, it would be possible, given that nothing like natural disasters, plagues, treachery, or any cause of resource shortage happened over that duration.

All the kingdoms have an armed force enough fight against the armies of darkness, but there were no threat against them for almost 500 years. The only treat they face are localized like bandits and wild beasts.

This is the most implausible part: that the kingdoms should pay to maintain a standing army which warms benches for 500 years. If you replace the standing army with some kind of militia then you remove that objection and also reduce the risk of war between the kingdoms. If I'm already paying a standing army, it's economically worthwhile to raid my neighbours for booty to pay them; if I would have to raise the money to recruit and train the army before going to war then the cost/risk balance changes considerably.

Note: some historical feudal systems had a kind of compromise, where the feudal underlings had to provide so many man-days of military service per year. You want to avoid this for the same reasons.

When it comes to an attack from the armies of darkness, it shouldn't be difficult to motivate the population to take up arms. They won't be as well equipped and trained as if you had a standing army.

Since the kingdoms do not have an external danger, they would fight between them, unless certain conditions are met.

• Each kingdom might be tied to its territory due to biological reason, so they are incapable of successful living in other territories. For example, a deadly disease would prevent invasion because the invader does not have resistance to that disease. An historical example is malaria, that prevented european powers to colonize sub Saharian Africa for many centuries.


• Certain animals or food might be regionally located due to weather or soil, that would prevent colonization. For example, Bantu expansion in Africa did not reach Kalahari desert, hence, people that lived in the desert was not conquered by bantu people.


• Transport is controlled, like the example of Dune world, where no war was possible without the help of the Guild's navigators.


• Distance. If distance between kingdoms is huge, is hard to move an army to invade another kingdom.

They simply need to have strong incentives/influencers to cooperation and to ensuring mutual survival that do not tend to push towards fusion.

Various forms of this include...

• Having a shared religion with a shared religious organization across all five countries that, for whatever reason, benefits from the status quo.




• Something about magic, prophecy, and the armies of darkness that actually blesses the ruling families of each kingdom, and gives each fo them reason to want the others around (because when the armies of darkness come knocking, you want to be sure you have access to the whole set). Bonus points if it's in some way tied to being the rulers of their respective lands (associated with certain immovable artifacts or whatever)

It also helps to remove the chances of claims on each other's land.

• Clear, unmoving natural borders. Bonus points if they're fantasy races who have no real use for each other's lands. If one (and only one) of your kingdoms is made up of mostly-aquatic people who live on the near oceanic shelf, they're not going to be trying to seize your lands any more than you want to try to seize theirs.




• Some way of preventing people from having both meaningful-but-contested claims on Kingdom A and family in Kingdom B




• Some clear, well-respected, objective way of determining succession in general




• Some massive increased cost of warring between the free people. If every battlefield turns into a place of madness, corruption, and crawling undead, then people will be more inclined to go for peaceful solutions.

My answer is based on the assumption that the time period is medieval or earlier. You define "close relations" as meeting every 5 years. This implies there is no electronic communication or phones, etc. And that transportation is slow. Other things you say (and mostly what you don't say) point to this as well.

Instead of looking to which modern nations (spoiler alert: none) have been at peace for 500 years, go back in time to find small nations that got along for prolonged periods. Remember too that what was called "kingdoms" way back then, were often what we would call cities now, with perhaps a day's travel between them. Not always of course, and there were early empires, but often.

We see some candidates in West Africa. While these kingdoms tended to engage in wars and the like in the 11th century on, it appears that things were a lot more stable in the centuries before then. With some of the kingdoms existing since 800 CE.

Aksum in East Africa was pretty stable from 1-600 CE. Ghana in West Africa (also discussed in the above link) began in 300-450 CE and lasted many centuries. And many others. Presumably, there were multiple smaller entities one could call kingdoms that traded with each other reasonably peaceably.

In what is now Peru, the Norte Chico civilization flourished between the 30th and 18th centuries BCE. Several others existed for long periods. We can't conclude that this means they were at peace with their neighbors (or with smaller entities within their borders), but it implies there was stability of some sort.

Then there was the Iroquois League of Nations. Five nations working together since perhaps as early as 1142 CE. In the early 18th century, a 6th nation joined them.

The Iroquois Confederacy is believed to have been founded by the
Peacemaker in 1142, bringing together five distinct nations in the
southern Great Lakes area into "The Great League of Peace". Each
nation within this Iroquoian confederacy had a distinct language,
territory, and function in the League. Iroquois influence at the peak
of its power extended into present-day Canada, westward along the
Great Lakes and down both sides of the Allegheny mountains into
present-day Virginia and Kentucky and into the Ohio Valley. The
League is governed by a Grand Council, an assembly of fifty chiefs or
sachems, each representing one of the clans of one of the nations.

We can pick apart any one of these examples. I'm sure none of them lived in perfect harmony with their neighbors. But, yes, what you're asking for is possible. In the absence of a more powerful kingdom whose goal is to conquer, smaller states can live pretty harmoniously if they choose.