Battlefield 2's first expansion pack, Special Forces, first began its development sometime during or shortly before the release of the original Battlefield 2 by DICE Canada: Battlefield producer, Mike Doran, commented in August 2005 that "The truth is that work on Battlefield 2: Special Forces began several months ago.". It was officially announced on July 14, 2005 and released on November 21 of the same year. The focus of the development was infantry-based combat as opposed to vehicle-centric combat from the original. As such, most of the additional content in the expansion pack can only be used by or for infantry.
Link here