Must a retained combat card be reused in later combats, and does it apply beyond the combat it was played in?

Question

Accepted Answer

A combat card applies only to the combat it is played in. A card kept after a won combat may be used in a later combat that turn, but the player is not required to use it.

Paths of Glory

Must a retained combat card be reused in later combats, and does it apply beyond the combat it was played in?