We've never needed to convict anyone of a crime to kill foreigners not on U.S. soil. Never ever. At the time of the founding, it wouldn't even have been a question. Of course it's the sovereign right of the state to protect itself in that way. Of course the only people protected by the U.S. Constitution are those living within the scope of American jurisdiction.
And yet if that U.S. Citizen shoots an anti-tank rocket at an Abrams in Afghanistan he can be summarily executed by a Private First Class (to say nothing of a democratically-elected Commander-in-Chief).
There's definitely a discussion to be had about how and where a nation can decide to use military force in the interests of its self-defense, but citizenship has never been a barrier to military action outside of the U.S. in war time. See for example http://en.wikipedia.org/wiki/Ex_parte_Milligan
Now certainly the Supreme Court did not envision drone strikes and one-sided warfare outside of the United States when they talk about the laws of military government, but it does go to show that even as far back as 1866 that the law showed a distinction between military operations inside of the U.S. and outside of the U.S.
The criminal code is for maintaining civil order in areas under our jurisdiction. When the Constitution says that people have the right to due process of law, it presupposes that you're talking about a context where process exists (i.e. it is possible to bring someone to trial because you have police power over the area and functioning courts). It's non-sensical in a military context (and there need not be a declaration of war to have a valid military intervention--also a tradition as old as the republic).
> These assassination are occurring in countries that we're not at war with (and in some cases, never have been).
Sure, but the issue is two-pronged: 1) Whether the target is considered to be "at war" with the attacker and 2) Whether the state he's in allows the military action (or at least, doesn't consider it to be a causus belli).
The issue we were discussing is the first (i.e. whether it is appropriate for the U.S. President to put an American citizen on a "kill list" without trial). Many people claim that this is unusual behavior, but it's really not in the context of the military (even in the media, e.g. Apocalypse Now). Or differently, if an American officer had defected to the Nazis during WWII and was known to be in a certain bunker you can bet he would have been facing a bunch of incoming U.S. ordnance.
However speaking towards the second point for just a bit, the U.S. assassinated a Japanese Admiral (Adm. Yamamoto) during WWII and didn't wait for him to be in Japanese-controlled areas to do so. He was killed in international waters, and not even during a battle as we think of them. He was a militarily valuable officer in a military organization fighting the U.S. and off he went. And, even if he somehow managed to overfly some nation's airspace, he would have been hit as long as the nation involved would tolerate the incursion.
> Furthermore, many the citizens being assassinated have never even been accused of a crime.
Well, they have (insofar as the process as it's been released theoretically requires some form of justification). They haven't been accused in a courtroom, perhaps, but that's a civil law matter, not military/international law.
I want to stress real quick that I'm not even personally decided for/against on this issue myself, but I'm a firm believer in garbage in == garbage out and so I just wish to point out that this issue isn't as crystal-clear as it's sometimes made out to be. If we treat this as a military affair (and AQ certainly does) then there is plenty of reason to treat the U.S. reaction under military terms as well, and those are much muddier waters than civil/criminal law.
Painting Anwar al-Awlaki's death as the short step away from extrajudicial executions of U.S. Citizens on U.S. soil is nothing more than handwaving. Al Awlaki was not protected by the U.S. Constitution not because he was labeled an "Enemy Combatant" and thus magically stripped of Constitutional protections, but because he was a fugitive actively fighting against the U.S., on foreign soil, and evading every attempt to bring him to justice.
The U.S. has never required a judicial verdict when killing fugitives in the course of battle is concerned. There were some American Citizens who served in the army of Nazi Germany. Some of those were killed by allied forces. No judicial determination was necessary. The same thing happened with British Citizens defecting to Germany.
Even in more (relatively) mundane situations, U.S. Citizens can be killed by the government with no judicial authorization. If you're in a building holding hostages at gunpoint, you might very well be taken out with a police sniper's bullet, and there is absolutely nothing unconstitutional about it. The sniper does not have to go get some sort of judicial authorization to take the shot.
You can bring up the "slippery slope" fallacy all you want, but you think it was different at the founding of the republic? Less than a decade into it's existence, the U.S. sent federal troops to suppress an armed insurrection against whiskey taxes. Nobody got a court order to kill the insurgents: http://44363370.nhd.weebly.com/battle-of-bower-hill.html. You think the U.S. government circa 1789 would've done anything different with al-Awlaki?
With all that in mind, I'd still rather that people leaned towards "that's reprehensible" as a gut reaction to first hearing of it than the alternative. I think if you ask me on most days I'd say there's legitimate military reasons to be able to combat terrorism in this fashion (especially in this brave new world of near-stateless persons) but it's still the kind of thing that I would want people to be very sensitive about.
See: http://supreme.justia.com/cases/federal/us/317/1/case.html, http://supreme.justia.com/cases/federal/us/339/763/case.html