What if Israel were hit by a nuclear weapon and we didn't know who fired it?

In the modern world of 'non-state actors' this is no more far fetched than any of the other scenaria that have been floating around (not that that's saying much). We no longer live in a world where there are the kinds of clear, bright lines people are trying to draw - trying to draw those lines is what got us into Iraq, let's not do it again, and, certainly, let's not level Iran while thinking that way.

Iran - A year of two from now:

Ahmadinejad is still holding onto the presidency, but he's continued to lose influence and power to Rasfanjai's party. The Iranian economy is (still, further) in the tank and there have been street protests over the mullah's continued interference in the electoral process.

A nuclear bomb lands in Tel Aviv. First indications from satellite tracking is that it came from Iran. We don't know who fired it, the Iranian government says it wasn't them.

What does the U.S. do? Retaliate unilaterally with massive conventional force? Support Israel to retaliate with their own nuclear capacity? Kidnap the entire U.N. Security Council and fly them to Tel Aviv?

We probably don't send some guy in a golf shirt and penny loafers with no socks over there to 'make peace' - but diplomacy and statecraft include lots of options between that and starting WW III.

I believe that Senator Obama would use every bit of resource and skill he has to find the best solution he could - whichever option that might turn out to be. I believe that Senator Clinton would also consider options, but her political rhetoric would limit those options, and I'm not confident that she really believes that the options would work. John McCain might do OK, he knows something about war and misery, but he might also just get grumpy and decide that Tehran needs more parking.

There's a lot of space between a banner headline saying 'Obama says he would not defend Israel' and 'Obama says all options are on the table'; you don't have to go straight to 'massive retaliation'. That space that could be very useful (even 'expedient') at some future date.

we'd know who fired it before it landed.

but not who fired it. A reasonable tactical weapon can be fired from a large truck with a couple of supporting vehicles. It doesn't have to come from a military base. In my scenario it came from a point out in the puckerbrush someplace and the Iranians immediately say it wasn't them.

What do we do? Just tell them they're lieing and retaliate? Or do we try to do something to find out what's really going on?

Ireland would get hit because it would be 11pm and she'd be out of it.

Obviously.

After all, did it matter at all that 18 of 19 hijackers were Saudi's?