During chapter 5, civil war breaks out in New London. You are given three choices at the start -
- Banish the Faction that organised the assassination.
- Seek reconciliation between both Factions
- Enforce order through laws
If you select the 3rd option, it opens the path to you becoming captain. In the council, go to the "rule" section and start a vote on the "Captain's Authority" law. (You have to have passed all other "rule" laws before this.). Once "Captain's Authority" law passes, next time onwards, you don't need anyone's approval. Next time there's a "vote", you'll get all 100 votes and unlock this achievement.
I think becoming captain also unlocks this -