All commanders are one of three classes.
Success in combat depends on your Commander affinities. The three affinity types, Melee, Cavalry, and Ranged have differing levels of effectiveness against each other.
Melee: Strong against Cavalry, weak against Ranged.
Cavalry: Strong against Ranged, weak against Melee.
Ranged: Strong against melee, weak against Cavalry.
When starting a new mission, take note of the enemy affinities and adjust your squads for maximum effectiveness.