There has been some feedback regarding AI abusing the retreat mechanic. However, in my opinion the whole problem was not covered yet and short-sighted solutions were left standing.

The current state of the retreat mechanic can be absolutely game-breaking. It's not just an annoiance. 

You can completely halt an army and all armies within reinforcement distance by attacking & retreating with three alternating one-unit armies.

Depending on turn-orders, this will break any competitive multiplayer. (Also AI did this to me for 4 turns, so its an annoiance in single player too)

Spamming one-unit armies makes them invincible. You retreated from one enemy army and ran into another? Fear not, if you spam enough, this army can too be attacked & retreated from, with another of your little pests. If you have one army for each army your enemy has, independent of unit-count and strength, then you are free to ransack with any additional armies.  

There are no simple solutions either.

One obvious change would be to disallow attackers that initiated combat to retreat (whats the point of that anyway, you have all information before initiating combat.) But there would still be much room for stalling with cheap one-unit armies and running a kind of 1000-needles guerillia warfare that no human player would enjoy.

Another option would be to somehow pick what armies/units are part of a battle and making retreat only affect those. But this would be messy to realize in multiplayer and come with a lot of side effects.

Making retreating units suffer damage proportional to the strength of the party they retreat from (or outright forcing one combat round to occur even if retreating) would help alot. But with cheap enough units, even these losses may not stop a good spam. (Produce one unit per turn, sacrifice it in a battle and continue to stall armies that are close to the city forever.)

This last problem actually isn't even part of the retreat system any more. Combat stalls complete armies (and every army that could have reinforced) no matter how weak the enemy was. Produce one cheap, fast unit per turn and halt all movement within a (how large is the reinforcement region anyway)-radius region.

We may (in addition to the above changes) need some kind of "advance with force" mode that allows us to move armies, even if they are attacked (with or without retreat). Maybe this mode halves movement speed but makes those movement points uninterruptable. Yes this may result in an army fighting multiple combats within one turn, but thats exactly what we need to counter weak army spamming.

How about 5 global retreat charges (per era) that can be used by any army

Making the attacking not able to retreat, or alternatively, able to cancel battle without retreating, is enough. There already is a heavy morale penalty for retreating.

But you can still move after you have been attacked? Its much more annoying if you're the attacker and the opponent just blocks you with 1 unit stacks.

Uh....if you attack, the defender can still move...there's no issue here at all.

Once you retreat, you cannot retreat again. I use this known mechanic to kill the retreating army.

Sometimes retreat is even unsuccessful and you're destroyed.

The only thing that might be an issue is that the retreat distance is greater than move distance. Gameplaywise and logically, a retreating army shouldn't be able to flee as fast as pursuers.

I'd just allow for the Endless Legend mechanic of retreating with half max HP loss or so