Dune: Imperium · Board game rules
When Hagal has no troops deployed, does drawing Hagal card still trigger its effect?
Quick answer
No — during Combat, a Rival (including House Hagal) only reveals a card for its Combat bonus if it has at least one troop in the Conflict; otherwise it's skipped.
Details
The Combat phase rule specifically states that "each Rival that has at least one troop in the Conflict gets a Combat bonus," revealed via the top card of the House Hagal deck, using only the sword icons shown [p.19 — COMBAT]. If a Rival has no troops in the Conflict, it does not reveal a card or gain any Combat bonus.
This is separate from Agent turns: House Hagal reveals a card from its deck every time it takes an Agent turn (as long as it has an Agent remaining), regardless of troop deployment, since that reveal determines where it sends its Agent, not a Combat bonus [p.19 — AGENTS].
Watch out: Don't confuse the Agent-turn reveal (used to place an Agent on a board space) with the separate Combat-phase reveal (used only to grant swords to Rivals already in the Conflict) — troops in the Conflict are required only for the latter.