If the president doesn't have such legislative approval or a legitimate "imminent attack" rationale, he is essentially murdering people -- U.S. citizens or not.
February 10, 2015 "ICH" - Although U.S. drones firing missiles at suspected bad guys in faraway places -- such as Pakistan, Yemen, and Somalia -- have gotten much publicity in recent years, it was recently revealed that the CIA assassinated top Hezbollah terrorist Imad Mugniyah with a good old-fashioned car bomb in Damascus, Syria with President George W. Bush's strident approval in 2008. Because of an executive order, signed in 1975 by President Gerald Ford, prohibiting assassinations by the CIA, presidents usually get around that order by using the military to kill an enemy bigwig and then make the disingenuous claim that it was merely taking out a "command and control" target rather than an assassination. In this case, Bush, never one to observe constitutional or legal niceties, became incensed that the CIA director was being too timid in carrying out the hit using the exploding car. The real issue in such cases is not whether it is more dangerous to liberty to kill the enemy using a high-tech drone or a more traditional car bomb, but whether it constitutional to do either.
Even Judge Andrew P. Napolitano, the author of numerous books and a legal expert for Fox News, in an otherwise excellent history of the usurpation of unique American civil liberties at the expense of ever-expanding executive power (see Suicide Pact: The Radical Expansion of Presidential Powers and the Lethal Threat to American Liberty) focuses too much on President Barack Obama's killing of American citizens without due process -- for example, Anwar al Alawki in Yemen in 2011. Napolitano correctly argues that an American president is essentially claiming the right to murder his own citizens without prior legal niceties, but he focuses too much on the use of exotic drone technology to do so and not enough on a larger and more important problem. If anyone -- U.S. citizen or not -- is attempting to attack the United States, the president should have a right to take them out, provided the Congress has authorized military action or declared war. Even then, according to the founders' original constitutional vision, if the country is under imminent threat of attack, the president can take appropriate action and get congressional authorization at the earliest possible time. If the president doesn't have such legislative approval or a legitimate "imminent attack" rationale, he is essentially murdering people -- U.S. citizens or not.
However, Obama did not start the congressionally unauthorized drone wars in Pakistan, Yemen, and Somalia -- which because of this lack of legitimate authority are essentially murdering people without due process -- George W. Bush did. But Obama has accelerated the illegitimate killing. Neither president can be given an exemption for imminent attacks; the Pakistani Taliban in Pakistan, al Qaeda in the Arabian Peninsula in Yemen, and al Shabab in Somalia have been around a long time, so both presidents have had plenty of time to get congressional approval for the drone wars.
Of course, both presidents would claim that the post-9/11 Authorization of the Use of Military Force (AUMF) gave them approval to go anywhere in search of al Qaeda-related groups. However, upon reading the AUMF, that is not the case. The AUMF merely authorized the president to go after those who perpetrated or supported the 9/11 attacks or who harbored the attackers; thus, the authorization would be limited to the central al Qaeda group and the Afghan Taliban that had harbored them. Groups only loosely affiliated with al Qaeda, and that had nothing to do with the 9/11 attacks -- such as those mentioned in Pakistan, Yemen, and Somalia -- are clearly outside the purview of the AUMF. In other words, when Obama killed an American citizen in the main al Qaeda group, such as Osama bin Laden, it was not murder, because a legitimate war exists with that group. In contrast, Obama's killing of at least four Americans in the other theaters was murder -- but so is killing people of other nationalities, because he doesn't have the authority to do that either without due process (unless he can get Congress to approve military action in those places).
The only other drawback to Napolitano's thoroughly excellent coverage of expansion of presidential power, and it's related to this topic, is that he underplays the expansion of the president's war power at Congress's expense. Since 1950, during the Korean War, for the first time in American history, President Harry Truman declined to ask Congress to declare war to authorize a major military action. This unconstitutional usurpation of power by the executive is every bit as serious as Napolitano's meticulous documentation of presidential excesses in trampling of civil liberties at home during wartime.
The nation's founders placed the war power clearly in the hands of the people's branch of government, the Congress, to avoid the cancer that was running rampant then in Europe -- executives (mainly kings) fighting wars of aggrandizement at high cost, in blood and treasure, to the common citizen. Thus, if the president wants to kill enemies abroad, any killing -- whether it's done with drones, bombs, or bullets or whether it kills Americans or foreigners -- should be regarded as illegal and unconstitutional unless it has been approved by the legislative branch. Yet even this legitimately legal option might be used too much. Let's treat suspected terrorists as criminals, not warriors, and first attempt to use law enforcement cooperation among nations to capture and try them, using acceptable standards of due legal process. Using the criminal justice system as a first resort, and congressionally approved military action only as a last resort, would also avoid retaliatory blowback terrorism in response to the now profligate and illegal U.S. armed intervention in far flung places.