Article 51 of the UN Charter clearly gives the US the power to defend itself against those who attack it, meaning, in this case, terrorists, al Qaeda, bin Laden, et al.
However, I’ve heard the case made (and that does not mean I agree with it) that bringing down the Taliban regime or otherwise attacking Afghanistan would be a violation of international law.
Here’s an essay written by a Mr. Gregory Fox that says the UN has said large scale retaliatory acts like the kind explained in this essay and, presumably, the kind we are about to undertake, violates international law.
I know, realistically, the UN probably isn’t going to try and stop us or condemn us. Good thing, too, cause I want the Taliban gone for more reasons than harboring terrorists. But, from a strictly technical standpoint, would be be in violation of international law?