That would come in the very rare possibility of a court case. The court might have to decide what active assistance is exactly. Participating knowingly, because there's the mens rea provision of law, and knowingly putting a cluster munition into the ground and shooting it is being actively involved. If you're just sitting back and saying that the Americans are over there doing that, then that's not a problem, but if you're saying that you need a cluster munition in this position, then you're actively participating in it. If you're helping transport the cluster munition for its use, then that's active participation.
I think the word “active” is a pretty good one.