Complex answer? If this man get's nuclear technology, I'd say it's safe to bet that we would be hit... or Israel would... either way, that would not be good...
Legally, we may not have much of a leg to stand on, but morally I feel we do... but maybe that's just me :0)
Oh, we've got all the legality we need in SJ Res 23. Congress authorized Bush to be the sole arbiter of who or what was involved in 9/11, and to take military action based upon his conclusions alone.
"International law," of course, does not exist. They can write up all the pretty little UN resolutions and pacts and treaties they want, but at the end of the day, the US government is only responsible to the US Constitution.