By positioning your own armies, you can direct the movement path of an enemy army if it is forced to retreat. Directing retreats is a powerful tool to help you destroy enemy armies, especially early in the game.
When an army is forced to retreat, the province it retreats to is automatically chosen from among adjacent provinces. A province containing no enemy armies is always preferred over one containing enemy armies, no matter what the size. Even a tiny army in a province will prevent the enemy from retreating to it, if there's any enemy-free province available.
Thus, you can control the province an enemy army will retreat to, by moving one of your own armies into all adjacent provinces except one. That one province will be the forced path of retreat of the enemy.
The more precise criteria for the adjacent province chosen for retreat are:
- Prefer retreat to where there is the least strength of enemy soldiers including rebels and natives. Natives count whether they are active or iconized. Cavalry and artillery have greater strength than do infantry so the importance is not simply numbers of troops.
- For equal numbers of enemy troops, controlled provinces are preferred.
- The highest base supply province (highest tax and friendly fortress) is preferred.
- Retreat to sea only if there is no land province for the retreat.
Thus it is possible to predict retreat paths even when there are armies in all adjacent provinces.
How can you get an advantage by directing retreats? Generally, you force a retreat towards a province chosen to be advantageous to you in battle, and then you send armies there to crush the enemy. The typical way is have a lot of cavalry, then direct an enemy retreat into a plains or desert province. In plains or desert cavalry have a bonus, and you can annihilate the enemy with their powerful shock. Cavalry also are easy to use to beat an enemy to the chosen province, since AI armies are rarely pure cavalry and thus require longer to arrive than your pure cavalry army.
Another way to take advantage of directing retreats is to force the enemy to retreat across a river or across a strait. This gives you a large defensive advantage if you can beat the enemy to the retreat province. Given that they have to get across a river or strait, you can usually get there first, even with infantry. It is sometimes possible to "ping-pong" an enemy army across a river repeatedly.
In straits, there's an even more powerful tool. Force the enemy to retreat across the strait, then block the strait with a fleet. (Note that the retreat cannot be plotted across the strait if it is blocked when the retreat is forced; so you'll typically want to have the fleet in a nearby port.) If the strait is blocked on the day when the army is scheduled to arrive, then it is turned into a ghost army, perpetually retreating but never arriving.
If you plan a battle in the retreat province, it's generally best to make it one of your own provinces or an ally's, even though it will probably get looted. The reason is to maximize the supply for your own armies there, while minimizing that for the enemy, thereby preventing losses from attrition during the battle.
One final way which you can sometimes hurt an enemy force is to direct it into a province in which it has low supply. In this case you don't necessarily have to get there first, or even at all, to cause significant attrition losses on the enemy.