I think this topic deserves some more discussion.
The scenarios (if used) would define how the game plays. Like mattman pointed out, there are 2 options, pre designated scenarios, "ok next game were playing scenario x, your the attacker, so you go away and tailor your force for that "mission". No problems with this, as any commander worth his salt would do exactly that.
Option 2 is to create a balanced list, then roll a die for the scenario to be played. I kind of like that too as it encourages players to think about all the difficulties they "could" face. It nicely replicates the uncertainties of war. Many times I have personally planned for rehearsed and briefed my men for an operation, only for it all to change at the last minuite, (the Heli isnt coming, your walking, or you have lost the machine gun section, they are going to A company instead....etc)
The bottom line is that I really like the idea of scenarios, its much more characterful, and is infinitely better than sending my little metal men to their deaths fighting for a cardboard counter that says "objective 3" on it
I will put some more thought into this and see what I can come up with.
Steve