Raw Story has a detailed 42-page defense of the President's "inherent constitutional authority" to conduct warrantless investigations of enemy forces to dissuade attacks upon the United States.

While I haven't read the entire thing in detail, it appears to my eyes that the document is a fleshed-out version of this five-page DoJ briefing (PDF) released December 22nd, and it seems like they're making the same points I discussed here.

The document cites copious case law, the President's inherent Constitutional authority under Article II, and a FISA exemption granted by the Authorization for Use of Military Force (AUMF).

It also makes the argument that if FISA is shown to conflict with the President's Article II Powers, then FISA is unconstitutional.

This is going to be very interesting, but I'd say unless the President's detractors can come up with a new argument I haven't heard of yet, then his powers to conduct this kind of warrantless surveillance will be upheld to the great consternation of those ts that do not understand the responsibilities on the Executive during a time of war.