One might add --- althought Greenwald does not mention it --- that President Clinton also asserted --- and exercised --- the unilateral power to attack a sovereign nation, that is, Serbia, against whom the United States and its NATO allies waged an air campaign for 78 days. Congress did not authorize the campaign. At the same time, I am not aware that President Clinton ever claimed that he would ignore an act of Congress that purported to countermand his decision to make war on Serbia.
In my view Greenwald overstates the equivalence between the position taken by President Obama (and Clinton) on the one hand, and that taken by President Bush, on the other. In some ways President Obama' claim of executive warmaking power is broader than that articulated, or at least pursued, by President Bush. But, there is also one sense in which President Obama's actual assertion of authority is less sweeping than President Bush's. Here is what I mean.