Independents
Basics
Most Dominions 3 games start with each aspiring pretender's kingdom controlling one or just a few provinces; the remaining provinces are neutral.
Just because a province is neutral, however, does not mean that it is undefended. Each province has a population type; this influences both what troops can be recruited locally (and these will be the only troops recruitable there, until a fortress is constructed to allow recruiting 'national' troops, or a magic site is found that provides other troop types), and what troops are likely to be found fiercely defending their autonomy. A province that offers human feudal troops, for instance, is likely to be defended by a mixed force of human longbowmen, heavy cavalry, and heavy infantry.
There are, of course, exceptions. These tend to be highly interesting provinces. :D In addition, should the force be missing or seemingly highly depleted, this may also be a curiousity...
Some forces are much stronger than others. Their territories may make interesting buffer zones, especially for nations with stealth, flight or other abilities that let them bypass them. Keep in mind, however, that your opponents could be doing the same to you, and that you can't patrol to detect enemy spies in a province that you don't control.
Behavior
Independent forces behave differently than normal AI-controlled sides. For instance, they are passive; they do not recruit additional troops, patrol, build buildings, invade neighbors, or so forth. The province defense of an independent province is normally zero -- this means that their army is not backed by 'free' militia that respawn until province control changes. If broken in battle and they retreat, they vaporize completely.
This passivity and lack of province defense does mean that casualties inflicted on them are essentially permanent. In addition, they lack the free commander of the province defense, which means that it is possible to assassinate -all- their commanders in a province. In this case, their leaderless force will break the next time they are in battle.
Only passive factors such as certain random events will replace any losses.
Passive Effects and Random Events
They are affected by random events; however, as Independents, events that cause independent forces to appear augment their forces rather than force battle.
They are affected by any magical sites that have an impact on all units in the province.
They are also affected by any passive effects of magical items. It has been known to happen that an independent commander starts with an item that automatically summons a free demon every turn...