Which Assassin's Creed game first introduced the Brotherhood feature for recruiting allies?

Assassin's Creed: Brotherhood first introduced the Brotherhood feature for recruiting allies.

Released in 2010, Brotherhood continues Ezio Auditore's story in Renaissance Italy. Ezio can recruit ordinary citizens who have been helped by the Assassins, train them, and call them into combat during missions.

The recruits can also be sent on assignments across Europe. Successful assignments improve their abilities and help develop the Assassin organization. This system made the idea of managing a growing Brotherhood a central gameplay feature rather than only a story concept.

Assassin's Creed II introduced Ezio and the Renaissance setting, but it did not include the same recruit-and-deploy Brotherhood structure. Revelations expanded Ezio's later story, while Assassin's Creed III introduced different systems and a new protagonist.