He never mentions international law and mentions the UN in passing as one of those "multilateral" possibilities that justify aggression. It is left up to his commenters to bring this up.
I think Article 51 is unambiguous:
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.
I think it is obvious that there must be an on-going/active attack. Not one that happened 6 years ago. Sorry, but 9/11 can not justify any current action -- such as, say, Obama's theories about Pakistan.
Dan Kervick comments, that Michael says he agrees with, does not fall within Article 51:
If my neighbor is shooting at my house, I am entitled to shoot back. If my neighbor goes out onto his lawn, loads his gun and takes aim at my house in a deliberate and aggressive manner, I am also entitled to shoot back. That's preemption: once it is reasonably clear that an attack is about to take place or is under preparation, I need not wait for the first blow to be struck.
The first sentence falls within Article 51. The second sentence is completely wrong. The UN and other treaties of the time were created with WWII in mind. State aggression outside of self-defense under an active attack is meant to be forbidden to states; and state aggression is designated as a war crime. All as a response to WWII.
... but we have to recognize that there may be times and places where it is appropriate and where America must defend its national security.
... while also recognizing that in exceptional circumstances it may be necessary to act preemptively - say for example the takeover of Pakistan and its nukes by Islamic extremists.
This is the same sort of justification pro-torturists make.
And finally this:
Shoot first and ask questions later is not a good way to run a national security policy and frankly is out of mainstream of our nation's history.
Which Arthur has sufficiently (already) responded to (and this series).