Rebels are armies not under the control of any country. They pop up in your country and fight you even when you're at peace. Rebels bear a flag and coat of arms that is half red and half black, divided diagonally.
For tactics to use on rebels, the wiki has an article on How to Handle Rebels.
Rebels can appear in the game from one of two causes. The typical way is rebellions: new rebels pop up on the map. Rebellions have a variety of causes including events, revolt risk, and failed religious conversion attempts. Note that rebellions typically occur on the first of a month. However, stability hits at -3 stability or any change to the religious tolerance sliders cause a rebellion check. The other way that rebels can be created in the game is in civil war, where a country's existing armies are converted into rebels.
Rebels in Battle
Against AIs, the rebel "country" has a land tech level that is low, similar to pagans, and low morale. Thus, AIs tend to rather easily beat rebels in battle. (This was also true for humans up through EU2 1.07; it was changed in 1.08.)
When fighting your (human-controlled) country, rebels are given the same morale and land tech level as you have. So you are basically facing even odds for a straight-up fight. If you attack rebels in mountains or forest, or across a river, they have a superior DRM to you and you're likely to lose unless you've got favorable odds. It also appears very strongly that there is some sort of advantage given to rebels in battle vs the player, such that they are quite hard to beat even in a fair fight.
Unlike other armies, rebels never retreat. Instead, when their morale fails in battle they disappear. They also will be eliminated for inability to retreat if their numbers fall below the level required to cover the province they are in. This often happens when they are left alone in a province that gets winter, after a winter or two. But you can also use it when fighting rebels. If you are fighting rebels in a province with a non-rebel controlled fort, you can safely retreat as soon as they drop below the cover level. You'll lose some victory points (if you care about that), and you may take 1% movement attrition. But you'll save the men that would have otherwise been killed in the last few days of combat it would have taken to kill off the last rebels.
When they appear, rebels always try to capture the province they are in, then move on to a new province and take control of it, etc. They'll do so indefinitely. Their choice of next province is not random. Rather, rebels try to capture a whole nation (as defined by "minimum" and "extra" sets in revolt.txt).
When rebels are in a rebel-controlled province, they do one of two things. If there are multiple stationary rebel armies there, they take the month to reform into one army. Otherwise, the rebels will select a new target province and start moving there.
Rebel movement is recomputed on the first of each month, and also upon loading the game, but only for rebels in rebel-controlled provinces. If conditions change while they're moving, rebels in rebel-controlled provinces may change their target (and thus change their move).
Exactly how do rebels choose their target province? Rebels only target colonized provinces (even a trading post), and they never target other rebel-controlled provinces. Keeping those general rules in mind, the decision works as follows.
First, using their current province as a guide, rebels choose a "captive nation" to try to free. (This affiliation is strictly temporary, only for the duration of the decision of where to go next.) The captive nation is as defined in revolt.txt, and is chosen based on these criteria:
- the captive nation must not currently exist. (A nation's start date and expirydate definitely do not affect rebel decision making. I don't know whether rebels observe any other conditions on nation formation encoded in revolt.txt.)
- the rebel province must be in the nation's "minimum" province list
- if more than one such nation exists, take the one defined earliest in revolt.txt (that is, the one closest to the start of the file).
When a captive nation has been found, rebels choose their target from among its "minimum" and "extra" provinces. If there's no captive nation, the initial set of provinces considered will be all adjacent provinces, plus all provinces owned by the country owning the rebel province. The initial set of provinces is then refined according to the following set of criteria, listed in order of priority:
- uncolonized provinces and rebel-controlled provinces are never targeted
- if there's a captive nation: its "minimum" provinces are preferred to its "extra" provinces
- adjacent provinces are preferred to non-adjacent
- [if no captive nation:] provinces owned by the owner of the rebel province are preferred to provinces owned by other countries
- wealthier provinces are preferred to less wealthy. "Wealth" is the left number you see on the city screen or the "province income" number on the province screen: modified Base tax + production + trade taxes + trade tariffs + manufactory bonus. Decimal numbers are rounded.
- if there's still no unique target province, target the province with the lowest province number (as in province.csv).
Note that government collapse is checked before rebel movement. Thus it's very unusual to be in a situation where the owning nation has no valid province to target.
If rebels target a province they can't actually plot a move to, they'll just sit there, unable to move. This often happens to rebels on islands, but you'll also see it in the New World where tiny groups of rebels "want" to move to attack a rich province that has fortifications, but the rebels are too few in number to even cover and thus cannot move in.
When rebels target a non-adjacent province, their move path often takes them through non-rebel-controlled province(s). Non-rebel armies do not affect rebel's target choice, but they may affect their path. If they fight a battle while moving, rebel armies do exactly the same thing your troops do: they continue their move only into the next province. If that province is not rebel-controlled, the rebels will then start a siege.
Rebels are subject to terra incognita just as players are. Rebels initially know about all of Europe, most of North Africa, and nothing else. Contrary to what one might imagine, rebels do not get maps of a province when they pop up there. They can discover new provinces in only one way: by moving into them. Rebels' target selection is not affected by their map knowledge, but their movement time is. Entering an unmapped province takes a very long time, for rebels just as it does for players. Note that unlike players, rebels may plot a move into unoccupied terra incognita; it just won't complete for them.