Nowhere. There are lots of laws on the books that aren’t mentioned in the Constitution. That’s because the Congress, according to the Constitution, has the power to make laws deemed necessary. There is, for example, the War Powers Act of 1973. This law prescribes the balance of power between the president and Congress in declaring war. The act requires the president to inform Congress within forty-eight hours of military action in a hostile area. Forces must be removed within sixty to ninety days unless Congress approves of the action or declares war.
In terms of U.S. law, there was absolutely nothing illegal about the second Iraq war.
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