This was always the question I repeatedly asked of Bush supporters who embraced this same War on Terror theory to justify all of his claimed powers: how can any cognizable limits be placed on that power, including as applied to US citizens on US soil (and indeed, the Bush administration did apply that theory to those circumstances, as when it arrested US citizen Jose Padilla in Chicago and then imprisoned him for several years in a military brig in South Carolina: all without charges). They did so on the same ground used by Obama now: the whole world is a battlefield, so the president's power to detain people as "enemy combatants" is not geographically confined nor limited to foreign nationals.
Out of the good grace of his heart, or due to political expedience, Obama may decide to exercise this power only where he claims capture is infeasible, but there is no coherent legal reason that this power would be confined that way. The "global war" paradigm that has been normalized under two successive administrations all but compels that, as a legal matter, this power extend everywhere and to everyone. The only possible limitations are international law and the "due process" clause of the Constitution - and, in my view, that clearly bars presidential executions of US citizens no matter where they are as well as foreign nationals on US soil. But otherwise, once you accept the "global-battlefield" framework, then the scope of this presidential assassination power is limitless (this is to say nothing of how vague the standards in the DOJ "white paper" are when it comes to things like "imminence" and "feasibility of capture", as the New Yorker's Amy Davidson pointed out this week when suggesting that the DOJ white paper may authorize a president to kill US journalists who are preparing to write about leaks of national security secrets).
That this is even an issue - that this question even has to be asked and the president can so easily get away with refusing to answer - is a potent indicator of how quickly and easily even the most tyrannical powers become normalized. About all of this, Esquire's Charles Pierce yesterday put it perfectly:
"This is why the argument many liberals are making - that the drone program is acceptable both morally and as a matter of practical politics because of the faith you have in the guy who happens to be presiding over it at the moment -- is criminally naive, intellectually empty, and as false as blue money to the future. The powers we have allowed to leach away from their constitutional points of origin into that office have created in the presidency a foul strain of outlawry that (worse) is now seen as the proper order of things.
"If that is the case, and I believe it is, then the very nature of the presidency of the United States at its core has become the vehicle for permanently unlawful behavior. Every four years, we elect a new criminal because that's become the precise job description."
That language may sound extreme. But it's actually mild when set next to the powers that the current president not only claims but has used. The fact that he does it all in secret - insists that even the "law" that authorizes him to do it cannot be seen by the public - is precisely why Pierce is so right when he says that "the very nature of the presidency of the United States at its core has become the vehicle for permanently unlawful behavior". To allow a political leader to claim those kinds of of powers, and to exercise them in secret, guarantee chronic criminality.