actually ... international law is something of a myth. it's only enforced by the will of nations and their ability to enforce it. it is illegal to attack another country w/o declaring war. but ... no one declares war before attacking.
the us will retaliate. that's a given. we hit afghanistan and sudan a while back. we've been bombing iraq almost daily for the past ten years. we've tried assasinations of various political leaders over the years. i only fear the fallout of how bad the retaliation will be ... but it will come ... and it will be brutal.