Which Assassin's Creed game was the first in the main series to include competitive multiplayer?

Assassin's Creed Brotherhood was the first main-series Assassin's Creed game to include competitive multiplayer.

Released in 2010, Brotherhood added online modes in which players hunted one another through crowded environments. Participants selected targets, used disguises and blending, and tried to perform stealthy assassinations while avoiding pursuers. These modes adapted the series' single-player social-stealth systems to player-versus-player matches.

The multiplayer component was separate from the main story, which follows Ezio Auditore in Rome and his efforts to rebuild the Assassin Brotherhood. Multiplayer characters included templar-aligned pursuers and specialized assassin-like figures.

Assassin's Creed II had no competitive online multiplayer, although it introduced important systems later used by the series. Revelations expanded Brotherhood's multiplayer framework, but it was not the first game to introduce it. The question concerns the main game series rather than mobile or spin-off releases.