Which Assassin's Creed game introduced the Brotherhood's recruit-and-train system?

Assassin's Creed: Brotherhood introduced the Brotherhood's recruit-and-train system.

In Brotherhood, Ezio can recruit civilians in Rome and develop them into Assassins. Recruits gain experience through missions and can be called upon to assist during selected activities. This mechanic turns the Brotherhood from a story concept into an active game system, giving players a strategic way to support Ezio while rebuilding the order in Rome.

Assassin's Creed II establishes Ezio's personal training and his relationship with the wider Assassin tradition, but it does not use Brotherhood's full recruit-and-deploy structure. Revelations expands Ezio's later story and includes related systems, yet it follows the feature's introduction rather than originating it.

Brotherhood was released in 2010 and is set primarily in Rome. Its focus on one large city, faction management, and recruitable allies helped distinguish it from Assassin's Creed II's multi-city structure.