Does an enemy have to be declared "officially" to be one? If so, we ain't got many enemies under Obama, except maybe Tea Party types, racist sexist homophobic rednecks. But no foreign ones.
I would imagine a prosecuter would have to prove that the ones aided or comforted were enemies, but otherwise, what's so special about them being declared so ahead of time?
There have been so few people actually successfully prosecuted for treason (well short of 100), it's really a guess. But, I would imagine the prosecution would like to enter into evidence something that objectively identified the foreign group or country, as an enemy.
If you read the Adam Gadahn indictment, it makes specific reference to the fact that he's alleged to be a member of Al-Qaeda, and that Al-Qaeda has been named as a foreign terrorist organization.