Which Assassin's Creed game introduced the Brotherhood recruitment system?

Assassin's Creed Brotherhood introduced the Brotherhood recruitment system.

The 2010 game lets Ezio Auditore recruit civilians in Rome, train them as Assassins, and call them into combat or send them on missions across Europe. Recruits gain ranks and abilities as they complete assignments, turning the Assassin order into an active strategic system.

Brotherhood continues Ezio's story after Assassin's Creed II and focuses on rebuilding the Assassin Brotherhood in Rome. The game also adds features such as guild management and multiplayer, making the organization itself a central part of the campaign.

Assassin's Creed II includes allies and story companions, but it does not use Brotherhood's formal recruitment-and-training system. Revelations expands the idea with its own systems but introduces it later.