Which Assassin's Creed game introduced the Brotherhood recruitment system that lets Ezio call allies into combat?

Assassin's Creed: Brotherhood introduced the Brotherhood recruitment system that lets Ezio call allies into combat.

In Brotherhood, Ezio can recruit civilians in Rome, train them, and assign them missions to gain experience. Once advanced enough, these recruits can be summoned during encounters to distract or defeat enemies, creating a major tactical expansion of the earlier games' stealth systems.

The recruits also represent Ezio's effort to rebuild the Assassin Brotherhood after the Borgia assault on Monteriggioni. Their missions take place across Europe and provide resources and progression beyond the main storyline.

Assassin's Creed II established Ezio's character but did not include this full recruitment-and-summoning mechanic. Later games developed related systems, including naval and settlement management, but Brotherhood is the entry that introduced this particular feature.