How do you unlock more army slots in Dragonfire?

The number of armies you can field (slots) goes up with the Army Expansion research, in the Stronghold's War Tactics tree. You start with few slots and open up to 5 armies by investing in it.

Worth knowing:

• It's a Stronghold research, so it RESETS every reign (the 8-week season). You re-grind it each season.
• The goal: get your 5 slots open, then keep them full. An army left undeployed is power asleep.

Don't skip this one early in a reign: going from 3 to 5 armies is +66% dragons on the field, so far more attrition dealt and more waves onto the same target.