First, if you can, get an army to their location as soon as possible. Having a rogue army appear in your 'backlines', near undefended settlements with none of your armies nearby, is significantly more problematic. Unless the rogue army has advanced units, this alone can usually tip the balance in your favor, so rogue armies near front-line settlements or heavily-fortified settlements are usually easily taken care of if you can get an army to their location. Since they are normally drawn to settlements, it is usually easy to engage a rogue army in a place where the settlement's garrison can provide reinforcements. This propensity to avoid armies and attack settlements means that rogue armies are very vulnerable to battles in which the enemy (you) receives reinforcements. Yet if the army feels that it has a decent chance of winning a battle (rather than a siege), it may begin raiding in the province or right outside the settlement. A settlement with an army embedded into it, especially a medium or large army (8-10 units or more, especially good units) will rarely be attacked.

A level II minor settlement with a small garrison will likely be attacked a level II minor settlement with fortifications and a larger garrison will probably still be attacked, but not if the rogue army has been significantly weakened by battle. However, rogue armies, like all armies, take local power into account. However, they are actively drawn towards lightly-defended settlements, usually minor settlements, which they will happily attack if the army has a significant advantage. Unless rogue armies are at war with you, they tend to avoid targeting armies.

Factions total war warhammer 2