Indeed, the administration has given themselves the authority in context of congress declaring war against al qaeda. Which means that congress could end the declaration of war and end the administration's new power. So perhaps I should have said that congress has in effect given the executive branch enough leeway to create this power, but in the end the power is still there.
Congress hasn't done it 'in effect', it did so very explicitly:
Now, therefore, be it resolved by [Congress]:
[Section 1 is the title of the act]
Section 2 - Authorization For Use of United States Armed Forces
(a) IN GENERAL- That the President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.
It's not the case that Congress declared war and the executive branch granted itself authority by extension. The phrase 'be it resolved ... [t]hat the President is authorized...' is the grant of authority.
A lot of people think this is so broad as to amount to a blank check, and I agree. But neither the Bush nor Obama administrations have have had to perform any legal conjuring tricks to derive this authority. Congress gave the President the authority in very clear terms. The President's authority to determine who is or is not a target under this legislation is absolute; it's not subject to qualifications or review by other branches. The language of the legislation is about as plain as it gets.
To piggyback on, the AUMF also explicitly mentions that it is the Act required in accordance with the earlier War Powers Act. They really leave it crystal-clear that the AUMF is very much intended to give the President the "Commander-in-Chief for a declared war" powers that Congress is able to grant.
It's horribly over-broad and open-ended, but Congress has only themselves to blame (and they can rein in the Act if they wish as well, or at least make the President try to veto it).
