It's illegal (1) against civilians, (2) against military targets in concentrations of civilians if delivered by air attack, (3) against military targets in concentrations of civilians by non-air attack unless the military targets are separated from the civilians and you take all reasonable precautions to limit incidental damage to the civilians, and (4) to use it on forests or plant cover unless they are being used to cover or conceal or camouflage military targets.

I don't think I'd describe that as "generally illegal" since it leaves a heck of a lot of situations where it is legal to use it.