Each stack must conduct operations separately. Thus, you can't move a stack up next to a enemy stack and attack with any units already in that hex. Since new units must be placed no more than one per hex, that makes concentrating difficult, particularly since new units must be placed in their home areas. Combat is very simple. A simple Odds ratio is computed and the die rolled, with results varying from attacker eliminated to defender eliminated.
Stratagems also can influence combat.
Wining gains the winner one or two stratagems. The defending force sometimes can withdraw, if it is faster than the fasting attacking unit, and combat does not always destroy the entire attacking or defending stack, which may instead have to retreat. Leaders, except for Charlemange, are obtained as stratagems. Pillage is an operation, but strangely, it is not automatic. Success depends upon the strength of the force, and may give the other player a stratagem marker even if not successful.