Armies are the base component of military power of Dominions. They're composed of any number of commanders using their leadership abilities to carry units around.

Early you might use small bands of elite sacred units blessed by a priest to give them extra battle prowess to expand, later you might have hundreds of units led by a few commanders, each squad with its own set of orders, backed by dozens of mages buffing them in battle.

A big decision point in Dominions strategy is knowing when to concentrate armies.

Concentration of forces in a single province comes with huge vulnerabilities that include:

• Remote Attacks such as Flames from the Sky or Murdering Winter
• A super combatant teleporting on top of them in one heroic feat
• A smaller army using spells like Foul Vapors or Fire Storm to inflict catastrophically disproportionate damage.

This makes doomstacks, i.e. stacking a lot of forces together in one unbeatable blob, not always a great choice. Multiple small parties will also be more efficient at raiding enemies and taking lightly defended provinces.

• Siege and taking enemy forts, or holding your own walls. Sometimes you'll hear about "one turn popping" a fort, meaning siegeing them down in one turn and being able to storm it down the turn after that, giving limited opportunity for the opponent to react.
• Defeating smaller armies in detail.
• Matching other big armies in battle, particularly if you don't have a mean to fight them asymetrically.

In general, big map move, as well as abilities like flying or terrain survival, is very valuable for giving you options to split and rejoin armies at will, potentially surprising enemies.