The U.S. does not need to declare war to be engaged in hostilities, nor does the U.S. need to be in a declared war for people who take up arms against the U.S. to be enemy combatants. This is a practice that is as old as the Republic: http://en.wikipedia.org/wiki/Declaration_of_war_by_the_Unite...

Holder's analysis was based on the AUMF, which as a Congressional authorization of force is as much as the U.S. has ever required.

As an aside, a declaration of war is a diplomatic issue between nations. It's not even applicable in this context, in hostilities against non-nation entities.