After getting a few cruisers in place, I was unable to force fire on an enemy's base. I could clearly see the targets, but the cruisers would not fire. After my ally's fly spy plane over the enemy's base, I could briefly target and attack the base.
Weird, I'm pretty sure cruiser's are supposed to be able to force fire, and I could in a single player test game I just tried, probably a freak bug/you were doing something wrong.