> Yemen is not a war zone in any sense of the word. There is no declared war on Yemeni soil. You could call it a "conflict zone", a term that could be applied to the vast majority of the developing world.

I don't understand this preoccupation with "declared wars." Who cares what you call it? The government simply does not have control over vast swaths of Yemen, and militant groups unaligned with the government are using it as a base of operations for attacks on people outside of Yemen. No, this does not characterize the vast majority of the developing world. Places like Yemen, Pakistan, and Somalia are in a totally different situation.

> Terrorists are really bad criminals. Are you able to draw a bright line between the two? If not, then what's the use of the distinction?

The line doesn't have to be bright, but it's crucial to draw the line. There is crime, and there is war, and the two are not the same thing. The regime applicable to external threats versus internal threats is very different. Maintaining this distinction allows you to better preserve the protections applicable to domestic criminal prosecution. If you try to shoe-horn well-financed international terrorists armed with military grade weapons into the framework of domestic criminal law, you end up distorting that framework and militarizing the domestic law enforcement apparatus. That's bad.

> He was never uniformed, he was not in a declared war zone, and he was not a citizen of a foreign nation.

You're elevating formality over reality. Al Awlaki wasn't unambiguously a foreign soldier, but aside from his U.S. citizenship he essentially acted like one. If a state affiliated actor had done the same thing, it would be considered an act of war.

> Am I mistaken?

Some procedural rights cannot be waived, and a constitutionally valid trial cannot be conducted without meeting that requirement. The historical understanding in the U.S. has been that trials in abstentia are unconstitutional, at least for capital cases. To date, the Supreme Court hasn't directly addressed the Constitutional issue (though it strongly implied it in Diaz v. U.S. in 1912), but has held that a trial in which a defendant is not present in the beginning of trial is defective for violation of Federal Rule of Civil Procedure 43 (Crosby v. U.S.)

From a practical viewpoint, trials in abstentia are a far more dangerous precedent than drone strikes. Sovereign countries already reserve the right to kill foreigners if they feel attacked by external threats. You can sugar-coat it with international agreements and treaties and whatnot, but fundamentally, every nation, as against external threats, embraces the sovereign right to kill people in other countries. That's the foundational basis for war.

The precedent for trials in abstentia in the context of domestic criminal prosecutions, however, is novel and scary in the U.S. and it's a stone better left unturned. It's better, for people back at home, to just say that people like Al Awlaki, through their long absence and actions abroad, simply relinquish their rights as U.S. citizens and can be treated as foreign enemies. Because you don't want the ramifications of the government figuring out how to deal with guys like him within the framework of domestic criminal enforcement.