Cornering the AI

How do you avoid this? I've seen it happen to me, I've seen it happen to the AI...mostly the AI actually.

In case the expression is a bit unclear then what I'm talking about is this:

Faction A (either player or AI) invades Province 1 (belonging to Faction B). Faction B (AI) desides "they cannot win the battle" and retreats without a fight. Sometimes leaving a few hundred men to certain death in a pointless siege (the AI hardly sallies or rescue their ridiculously big garrisons), sometimes not.
A - who now has the upper hand - continue in the same way for a few rounds, taking 2, 3 and 4 untill B is left with only 5.
Unless B has left more than half its troops to die in said sieges, it will now have three full stacks in 5, while the surrounding provinces 1, 2, 3, 4 and 6 (belonging to C) are left with one or less. B will now attack the seemingly outnumbered A or possibly C, either reclaimin one of 1, 2, 3 or 4 or taking 6. Sometimes B will even leave 5 unprotected, allowing A or C to finish the job and taking it, while B is now stuck with a new, single, province.

Now, if you are faction A this might be a big problem as you suddenly face almost the same situation as in case of a re-emergance; you have a lot of weak provinces surrounding one strong. The AI trapped in this province must move on as the lands cannot feed them.
Of course, chances are this isn't very much of a problem anyway - especially if this is in High/Late and you're playing as a catholic, in which case you can out-tech the AI, letting its inferior but numerically superior troops die at the hand of your...whatever. But if faction A is an AI-faction and even worse your ally, this can cause more problems than it might seem. This particularly evident in the Iberian and the Levant in Early.
Both the Almohad/Almoravids and Egyptians/Fatamids are potential superpowers that will often create themselves use empires using their über units, possibly at your own expense. The Byz will often do exactly this to the Eggies. They push them back a little, "corner" them somewhere in the desert or in Anatolia and then the Eggies use their superior numbers to completely turn tables on their adversaries, sometimes sending an army by sea to Nicaea at the same time.

Actually, this turned out to sound less of a problem than I first thought... hmm... You could just consider this an analysis if this is no big deal to you.

This is a classic strategic dilemma. Your objective is not only to seize the opponents territory, but to destroy or neutralise his armies. By pushing him into a corner, you allow him to concentrate his force.

One technique is to try and lure the AI out of the corner by leaving a weakened besieging force in province 3 or 4. If you switch your main army between bordering provinces, you can sometimes fool the AI into attacking your main army (or a substantial force at any rate). In a defensive battle you should be able to inflict heavy losses on the enemy.

Attacking a mega-stack is more of a challenge, but not impossible. Sometimes you will need to send in troops who can inflict damage without winning battles, e.g. horse archers, and allow the AI a pyrrhic victory or two. Use a crap commander who won't mind getting the "good runner" vice.

The AI tends to do what players do - amass armies at the border and leave the interior poorly defended. When attacking a large AI, I first sink its navy, and then make sure that its main army has nowhere to escape by attacking in multiple provinces. My main force attacks in the province where the AI has its main army, and I send smaller armies in the bordering provinces, if I can (preferably by sea). The AI tends to withdraw, and it does so, without realizing its escape route is threatened. It ALWAYS retreats from a losing battle if its ruler is in the attacked province, as it never leaves its ruler besieged - in fact, it avoids fighting defensive battles with its ruler even in cases it is likely to win. Then, realizing that if I successfully invade its neighboring provinces, it will have to ransom its main army (which often includes the retreating monarch), it accepts a battle it really cannot win against one of my smaller armies, and instead of fighting a huge battle I usually find myself having to fight a few units of Urban Militia or Spearmen with the odds overwhelmingly in my favor. Such a strategy is easy to pull off and effectively cripples the AI. I am afraid it might be classified as an exploit, but what else are you supposed to do - conquer a province after province, thinly spreading your forces just to give the AI a chance to counterattack?

If the AI has an unbalanced army, you can attack with an army of counter units which is numerically smaller than the defensive army. The machine will make it's calculations based on pure numbers, and you can cut right through them, slaughter the prisoners, and deny the enemy the chance to consolidate its forces after a retreat.

I'm sure that taking advantage of the logic (or lack thereof) the AI uses to decide whether to stand and fight is an exploit, but it is an option.

Whatever logic it uses is sometimes frustrating. In my Russian campaign the Hungarians were the only other faction close to my strength, and we spent a long time building our armies across our borders waiting for the attack.

Eventually I attacked, and we had pretty even numbers in several provinces. What did the AI do? Ran away! It happened again the next turn and after that too. The end result was that I grabbed about 5-6 provinces easily, they lost large chunks of armies in sieges and then descended into civil war for their king's cowardice.

Pretty bloody silly - I won the war with hardly a fight at all. Pretty disappointing too.

Strategically it was crazy, because you have to defeat the army in the field. Ceding ground can be a good move for many reasons, but eventually you have to pick your battlefield. In this case the AI was just calculating the individual battles without any consideration as to what it would mean strategically to retreat. In some of the battles I was attacking with half the numbers of the Hungarians just to get a fight, and even then they retreated... That was on vanilla though - not sure if XL has changed this. Alhtough I think that the AI retreats less in XL (maybe because its troop mix is always better?).

Yeah, it's kind of sad to se Saxon Huscarles stay under siege while I move forward to another and then another province, and then corner the now-diminished stack that might've given a big challenge...

I think that you could try to built a kind of "bait army" when leading your first attack, an army with which the AI accepts the fights, but will loose at least a lot of troops, or even the battle?

Unfortunately if you're not the one "cornering the AI", i can only imagine attacking the faction cornering the other? They tend to spred their troops when attacked on several fronts and are less likely to push someone else?

I support the above suggestion: launch a ranged unit strike force and go for the king, then withdraw. With luck they´ll go rebel and it will be easier to tackle with them.

After the first sixteen units on each side, additional units start mattering less. If you've cornered the AI with some big stacks in their last province, just arrange sixteen buff units with a few cavalry reinforcements and concentrate on winning the first leg of the battle. If you can kill/rout their general and deal severe damage (especially easy if the AI decides to try using siege engines against you) most likely their defense will fall apart and you can eat a lot of the rest of their stack even when it looks very impressive on paper.

Be sure to bring light cavalry though, to prevent rallies, especially if you happen to be playing XL with all the buffed up morale.

If you've cornered them into a corridor (for example, cornering the Russians in Finland while you hold Novgorod) then just leave a stack of eighteen or so and arrange a massacre defense.

If you´ve cornered the AI in the last province and send some buff units to kill their general, you don´t need to win the incoming waves, even if they´re 50, as they go rebel in the next turn anyway