Presidential Authority
The President of the United States (POTUS) takes an oath of office which says...
I do solemnly swear that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.
When enlisting, military personnel take a similar oath...
I do solemnly swear that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice. So help me God.
Along with the responsibility to defend the Constitution comes authority. The Constitution appoints the POTUS as Commander-in-Chief (C-i-C) of the military. As C-i-C, the POTUS is empowered to "defend the Constitution" implicitly... "against all enemies foreign and domestic".
So what is this Constitution that the POTUS and the military are to defend? A document? A piece of paper? Clearly not. The Constitution is more than a document, it is an idea. It is an organizing principle. It is a basis for governing. It is a blueprint for a nation. It is a way of life. It is in fact, that which "constitutes" America itself.
The Preamble to the Constitution reads...
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
When a foreign terrorist organization like Al-Qaeda upsets the "domestic Tranquility" by hi-jacking airplanes and flying them into buildings killing thousands of Americans, the Constitution provides "for the common defence" by authorizing the POTUS to act as C-i-C to direct military forces as required to eliminate the threat. When a foreign power is suspected of harboring terrorists and weapons of mass destruction, which might reasonably be used against Americans, the Constitution fairly cries out that steps be taken to "INSURE domestic Tranquility".
Of course, the entire government in all of its branches must work in unison to "insure domestic Tranquility". Congress must pass laws like the Patriot Act, that allow our enemies to be discovered and thwarted. The Judiciary must uphold those laws which the Congress passes, as well as the constitutional authority granted to the President. To do less, in my opinion, is an act of treason.
The POTUS however, plays a unique role as C-i-C during a time of war. He can conduct surveillance (including warrantless surveillance) on foreign powers and terrorists. This constitutional authority has been accepted as a matter of course by every President since George Washington. Robert Turner, in an excellent article on this subject Here, reminds us that the three branches of government are separate and co-equal, and that no law passed by Congress (including FISA) can limit the constitutional authority of another branch...
Every court of appeals that has considered the issue has upheld an inherent presidential power to conduct warrantless foreign intelligence searches; and in 2002 the U.S. Foreign Intelligence Surveillance Court of Review, created by the FISA statute, accepted that "the president does have that authority" and noted "FISA could not encroach on the president's constitutional power.
The President's constitutional authority to conduct warrantless surveillance on foreign powers and terrorists is not limited to electronic data-gathering (i.e., wire-taps), but extends to all forms of information including aerial reconnaissance, satellite imagery, human intelligence, interrogation of prisoners, and physical searches and seizures.
Does this mean that we should practice torture on prisoners to extract vital information? Absolutely not. And the stated policy of the U.S. military excludes torture from its interrogation procedures. But we must recognize that discomfort, hazing or psychological tactics does not constitute torture.
In a time of war, obtaining intelligence about the enemy is crucial to victory. Those who would try to deny our C-i-C the constitutional authority to do so, are at best on shaky legal ground, and at worst... border on treason.
UPDATE: For an excellent analysis which completely supports my point of view, go Here.