By the rules, a hero can take command of an army using his I-I with a TN equal to the number of units in the army. It's pretty clear with any reasonably sized army, it's just not going to happen, even for someone like Aragorn or Theoden. The best you can do is deal with the ones around you, so I was thinking, how about taking command one formation at a time? No more than one attempt could be made per turn, but you could slowly bring a leaderless army under command. It would still be difficult to impossible to bring really big formations under control (until casualties knocked 'em down). The attempt could even include multiple formations if you wanted to combine them (say you had a formation of 10 light archer units, and another of 10 medium archers -- combined for 20 units)