Mary Ellen O'Connell, in the latest issue of ILSA Journal of International & Comparative Law, argues that the war on terror does not comport with the understood definition of war in international law. She argues for a narrow definition of war and claims war should only exist "when compelled by the facts: facts of real fighting and situations of real emergency where normal peacetime law and protections cannot operate." The war on terror is too sporadic to fall within this definition.
Moreover, she claims granting Al Queda combatant status is detrimental to US interests by elevating them to equals with the US. Al Queda's prestige shot through the roof and that's a strong recruiting tool.
O'Connell's main criticism is that the administration wants this be a war so it can use execution without warning and detention without trial. Bingo. Of course that's why it does it.
The standard legal definitions do not lend themselves to modern factual situations. A terrorist war, launched by thugs who could obtain nuclear weapons, was not something envisioned by Grotius. And while the laws morph over time, they do so very slowly, way too slowly to allow countries to do what they have to do.
This is precisely why international law is of little utility during war. A country will serve its best interests in any wartime situations, only constrained by practical elements. The law is far in the back of its mind, and is only dredged up as support for a policy.
Policy arguments are the only way to persuade a country to take a preferred path, not vague statements about the "law."