I have 3 forces.
Force 1 = Humans
Force 2 = Comps (P7)
Force 3 = Comps (P8)

If I have a unit for P7 and a unit for P8 across from eachother (about 15 squares) and order them to attack once towards eachother, any reason why they shouldn't attack eachother?

I have no triggers allying them.

The computers always start out allied no matter what force their on. You have to change the alliance status.

Learn something new everyday .. lol thx.

The computers always start out allied no matter what force they're on. You have to change the alliance status.

Also, use the Patrol command, not the Attack command, computers don't attack unless something gets in their way or something when you use the Attack command.

Yea, that wont be a problem here.

What? I know I'm not going totally nuts.. I actually had to test it, but this is totally false. Controlled players (human or computer) on different forces are by default set to Enemy and will attack each other if not executing another order.

What Falkoner said is most likely the problem.

Ah, that did sound weird to me, but I tend to just set alliances as a safe-guard anyway, so I don't remember it well.

I was pretty convinced but I've been out of mapping long enough I had to make a test map to make sure.