Enemies of the United States can only exist during a formal state of war. Other acts such as aiding a foreign government are addressed through criminal charges of espionage, and more.

By law an enemy is ordinarily a foreign subject in some open hostility with the United States. There is no requirement for a formal declaration of war, though that does make the charge a lot easier to prove. For example, the AUMF functionally defined "enemies" without a formal declaration of war.

What law? In any event, what war? Mere open hostility by a foreign subject with the United States applies many people. That can't be the definition of "enemy" for constitutional treason purposes, because it would be overbroad and therefore contrary to the object and purposes of that provision.