I am not much of an FTM user but the way I set up military service is to add one custom EVENt tag in GEDCOM with a TYPE "military service" (aka event_descriptor) and the event_classification of the war/conflict name like "Civil War". I know that FTM uses different terms for these fields in the UI but the GEDCOM would look like this:
1 EVEN Civil War
2 TYPE military service
2 DATE FROM ... TO ...
If you want to record a specific battle substitute the word service with battle then replace Civil War with the battle name.
This the most correct GEDCOM way to record these events.