From what I've heard you play as multiple soldiers. I think there's three you play as.
One is a U.S. Army Ranger deployed for the "Big Military" operations.
The second is a Tier 1 operator. From what I know, these guys are selected from the best Rangers and Navy SEALS. They are shrouded in secrecy and sent in for undercover ops like: Sabotage, hostage rescue, recon, etc...
EDIT:
Last one if I remember correctly is an Apache helicopter pilot. In the trailers they show the player in one of those.