Actually, despite talk show discussion of the subject, assassination of a head of state is banned under Executive Order 12333. The Administration has gotten around the order, in the case of Osama bin Laden and his Al Qaeda associates, simply by treating them as terrorist combatants -- not as heads of state or equivalent (who would be subject to the protection of the anti-assassination provision).
So no, nobody would have done it already, at least not if sponsored by the US. However, if they were about to nuke Israel, I am pretty sure the Executive Order would be rescinded (quietly), making way for the strike force that is the subject of this adventure. It makes sense to me.