Iran and Israel
There is a fallacy in this argument. If our "national interest" in keeping Iran nuke free is the protection of Israel, then it is not at all unreasonable to expect Israeli action. Indeed, the rhetoric on mutually assured destruction does apply here - although not to the US. If Iran nukes Israel, there is no doubt that Israel would return the favor (or vice-versa). Of course, Iran doing so is unlikely, since both Iraq and Iran are downwind of Israel. Even if Israel could not get off a missile, Jordan, Syria, Iraq and Iran would all get fall-out from a strike on Israeli soil. It would also kill many Arabs (both Israeli and Palestinian). Only the most doctrinaire neo-con would ever think that Israel is at risk from an Iranian nuke (including one provided to terrorists - since that nuke would still kill Arabs).