Consider this.
Here is the relevant excerpt:
In other words, Congressional silence - especially in military affairs, where the President wears the mantle of constitutionally delegated Commander-in-Chief - enables him to act unilaterally. Thus, if Congress is silent about an attack on Iraq, which so far it has been, there is no doubt that under Justice Jackson's Youngstown analysis, President Bush has the power to act unilaterally.I encourage you to read the whole article referenced above as it details the case very acurately.It is another story entirely if the President acts against the express, or even implied, wishes of Congress. He is then at the "lowest ebb" of his power.Then, according to Jackson, the President can - rely only upon his own constitutional powers minus any constitutional powers of Congress over the matter. [e.g., the the President's Article II power as Commander-in-Chief, versus Congress's Article I power to declare (but not make) war].
In the case of Iraq, however, whatever Congress has already done to authorize the President's actions (see the Joint Resolution), it is manifest that the Legislature - unlike in Youngstown, where Jackson found that seizure was contrary to the will of Congress - has done nothing to oppose military action against Saddam Hussein's regime.
Just because they didn't cite it doesn't mean they didn't use it. Congress rarely cites it authority to do what it does. Of course that's often because it doesn't have any. Not true in this case however.