Within the U.S. collection of missions, I started off the Artillery Assault scenario, and was dismayed to see that I start with only three poorly-equipped soldiers to complete the mission with. This scenario is so tough and there are so many heavily-armed enemies, I could not last more than a couple of minutes.
Immensely frustrated, I turned to this walkthrough

http://www.gamefaqs.com/pc/915040-soldiers-heroes-of-world-war-ii/faqs/32520
This walkthrough describes how units will "carry over" from scenario to scenario in the US missions:
3.02) NIJMEGEN Remember: Even though your units from this mission won't carry over into the next mission, they will carry over into the mission after it. So don't think you can be sloppy with your troops. The more you have alive at the end of this mission, and the better they're equipped, the easier Artillery Assault will be.
3.04) ARTILLERY ASSAULT Any of the troops you had left at the end of the Nijmegen mission will be available for this mission, and they will still have any equipment they had at the end of that mission too.
When I read that the number of troops that you start with in the Artillery Assault scenario depends upon how many troops are left at the end of Nijmegen, I went all the way back to the Drop Zone scenario and played all the way up to the start of Artillery Assault. I made certain that I had six men, very heavily equipped at the end of Drop Zone. I think I ended the Nijmegen" with five men, and I ended Minefields scenario with at least eight men. However, when "Minefields" ended and "Artillery Assault" started up, once again I was stuck with only three badly-equipped soldiers!
WTF?

Other than a Wlakthrough I am not sure.
Maybe the game thinks you are so good this is all you need to complete the mission! :o)