If I read it right, the Act actually establishes a cause of action in any act of "international terrorism" in which "any national of the United States suffers injury in his or her person, property, or business." While there are at least half a dozen reasons a suit against Israel (in my hypothetical example) would fail, the Act is pretty expansive in where the acts can take place, and what kind of damage they do.

Well, the bilateral US-Afghan security treaty would bar filing in an Afghan court; in the US realm, you would have standing problems, the Baker bar, and sovereign immunity. If I recall -- and IANAL, let alone a nat-sec attorney -- there's some Fourth Circuit precedent that could justify stripping sovereign immunity from individual officials in jus cogens violations, but I don't think anyone else takes that seriously.

The bitter truth is that there's not much you can do in those circumstances. To paraphrase the Melian Dialogues, the strong do what they will and the weak take what they're given.