There is one mainline Assassin's Creed game before Assassin's Creed II: the original Assassin's Creed.
The first game launched in 2007 and introduced Altaïr Ibn-La'Ahad, the Animus framework, the Assassin-Templar conflict, and the series' blend of historical settings with science fiction. Assassin's Creed II followed as the second numbered mainline entry in 2009 and began Ezio Auditore's story.
Several related titles can make the count confusing. Mobile games, spin-offs, collections, and later companion releases do not create additional mainline entries between the original game and Assassin's Creed II. The wording “before Assassin's Creed II” refers to the numbered mainline sequence, not every Assassin's Creed product released in that period.