I started the game on recruit and now I'm on mission 5. When I try changing the difficulty setting, all I see is the "recruit" option (no other level is available). Has anyone else expierenced this?

I think that is because they do not want you to play through level 15 on recruit and then change it to the hardest to complete ...

From what it looks like it seems that you can go "down" to an easier mode but not "up" to a harder mode. I began the game in "Soldier" and when I go to the Change Difficulty section the only option is to go down to Recruit.

Edit: So it seems your best bet would either be to start the game over in a harder mode, or finish it in Recruit (getting a great feel for the whole game) and then replay it in a much harder mode where you will probably be better prepared for the challenge. . . I usually alway do a game the first time in Normal (Soldier in this case) and then go back and replay it in whatever the Hardest mode is.

Apparently you can only increase difficulty if you restart campaign. Other you can only decrease difficulty once you start.