Phased Withdrawal

While the term sounds like something akin to the rhythm method, the number of proposals for getting the hell out of Iraq continues to climb (see TPM Cafe article on Senator Barack Obama's recent proposal) . At this point only the furthest reaches of Wingnuttia see the fiasco as winnable. They also either refuse to define or more likely can not define what the now rhetorical term "victory" looks like.
The main hurdle to climb is of course "the decider". While the Constitution expressly names the President as Commander and Chief of the Armed Forces, only Congress can authorize military action. That is on a purely Constitutional basis. Subsequent additions to the U.S. code, commonly referred to as the "War Powers Act", see Public Law 93-148
93rd Congress, H. J. Res. 54 or, Title 50 Ch 33 of the US code, but more specifically Ch 33 Sec 1541(b) (c) states:

(b) Congressional legislative power under necessary and proper clause Under article I, section 8, of the Constitution, it is specifically provided that the Congress shall have the power to make all laws necessary and proper for carrying into execution, not only its own powers but also all other powers vested by the Constitution in the Government of the United States, or in any department or officer hereof.

(c) Presidential executive power as Commander-in-Chief; limitation
The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to
(1) a declaration of war,
(2) specific statutory authorization, or
(3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.

Does his "Mission Accomplished" speech, May 1st, 2003, constitute an end to hostilities? :

Admiral Kelly, Captain Card, officers and sailors of the USS Abraham Lincoln, my fellow Americans: Major combat operations in Iraq have ended. In the Battle of Iraq, the United States and our allies have prevailed. And now our coalition is engaged in securing and reconstructing that country.

Now couple the with Ch 33 Sec. 1543(c)

(c) Periodic reports; semiannual requirement

Whenever United States Armed Forces are introduced into hostilities or into any situation described in subsection (a) of this section, the President shall, so long as such armed forces continue to be engaged in such hostilities or situation, report to the Congress periodically on the status of such hostilities or situation as well as on the scope and duration of such hostilities or situation, but in no event shall he report to the Congress less often than once every six months.

HOWEVER AND THIS IS THE BIGGIE Ch 33 Sec 1544 (c) states:

(c) Concurrent resolution for removal by President of United States Armed Forces

Notwithstanding subsection (b) of this section, at any time that United States Armed Forces are engaged in hostilities outside the territory of the United States, its possessions and territories without a declaration of war or specific statutory authorization, such forces shall be removed by the President if the Congress so directs by concurrent resolution.

I am not a constitutional scholar or a lawyer, however it seems clear that if Murtha's, Obama's, or whichever resolution comes up for a vote and passes jointly than the president has no choice but to withdraw our forces.