Bandits and partisans attack your rear lines and your supply convoys. Cleaning operations will be necessary.
While bandits will be wiped out thanks to these operations, partisans are much tougher and better-organised : they will not be automatically removed with this kind of operation.
Bandits are created from fugitives from destroyed or worn-out armies.
Partisans are created from bands of bandits organising themselves. The chances of bandits turning into partisans depends on the date and the level of Barbary.
Bad news: Cleaning Operations are often dirty business and raise the Barbary score.