San Andreas is in crisis, an organization called R.A.S.A. (Revolutionary Anti-State Agency) dominated San Andreas. Few soldiers survived and the military power of the San Andreas is in the hands of this organization.
A meeting with the U.S. President was scheduled to solve the problem. The most important men in the U.S. were there, everyone there were talking about what they gonna do to regain San Andreas, when the Jacob Franco (CIA Chief) had an idea.
Jacob Franco offered the mission to Sergeant St. James and your new troop.

"St. James VS R.A.S.A." is a "mission pack". This mission pack is about St. James, a sergeant easily recognized by his heroism in the battlefield, he accept a mission, the most hard in your life. But he is not alone in his journey, two young soldiers are with him. Along the way they will have good companions in battle, some soldiers will die, others will survive.

If you wanna know more about the pack: http://www.gtaforums.com/index.php?act=ST&f=263&t=509532