For something to be definitively unconstitutional the federal courts would have to have the matter brought under their jurisdiction, and to have finally decided the matter. If this has not happened, the unconstitutionality of a process or series of acts would, I think, be a matter of opinion.
The constitution vests the power to declare war in congress, not the president or the senate. The War Powers Act of 1973
lays out the power of the president to commit U.S. forces to action. The five Presidents preceding G. W. Bush since 1973 have considered the war powers act unconstitutional and they and congress have frequently ignored it. So it does appear that Presidents and congress both frequently act unconstitutionally in the use of military force.
Since congress has not declared war, and a sovreign nation has been invaded, its government overthrown, and the country occupied based on what are now disputed facts as to the threat that country actually posed justifying the invasion, it seems to me that constitutionality is debatable. To call a soldier engaged in the occupation of that country a liar for opining on the unconstitutionality of the war doesn’t seem warranted. That congress voted on a resolution has no definitive bearing on whether that resolution was in fact constitutional. That fact would have to be established in the courts. It seems more reasonable to me to consider the man’s point of view based on his observations and logic rather than dismiss everything he has to say and calling him a liar because you disagree with a single opinion on a debatable matter.
Minus the findings prefacing the resolution, the act authorizing the use of force in Iraq reads as follows:
SECTION 1. SHORT TITLE.
This joint resolution may be cited as the `Authorization for Use of Military Force Against Iraq Resolution of 2002'.
SEC. 2. SUPPORT FOR UNITED STATES DIPLOMATIC EFFORTS. The Congress of the United States supports the efforts by the President to--
(1) strictly enforce through the United Nations Security Council all relevant Security Council resolutions regarding Iraq and encourages him in those efforts; and
(2) obtain prompt and decisive action by the Security Council to ensure that Iraq abandons its strategy of delay, evasion and noncompliance and promptly and strictly complies with all relevant Security Council resolutions regarding Iraq.
SEC. 3. AUTHORIZATION FOR USE OF UNITED STATES ARMED FORCES.
(a) AUTHORIZATION- The President is authorized to use the Armed Forces of the United States as he determines to be necessary and appropriate in order to--
(1) defend the national security of the United States against the continuing threat posed by Iraq; and
(2) enforce all relevant United Nations Security Council resolutions regarding Iraq.
(b) PRESIDENTIAL DETERMINATION- In connection with the exercise of the authority granted in subsection (a) to use force the President shall, prior to such exercise or as soon thereafter as may be feasible, but no later than 48 hours after exercising such authority, make available to the Speaker of the House of Representatives and the President pro tempore of the Senate his determination that--
(1) reliance by the United States on further diplomatic or other peaceful means alone either (A) will not adequately protect the national security of the United States against the continuing threat posed by Iraq or (B) is not likely to lead to enforcement of all relevant United Nations Security Council resolutions regarding Iraq; and
(2) acting pursuant to this joint resolution is consistent with the United States and other countries continuing to take the necessary actions against international terrorist and terrorist organizations, including those nations, organizations, or persons who planned, authorized, committed or aided the terrorist attacks that occurred on September 11, 2001.
(c) War Powers Resolution Requirements-
(1) SPECIFIC STATUTORY AUTHORIZATION- Consistent with section 8(a)(1) of the War Powers Resolution, the Congress declares that this section is intended to constitute specific statutory authorization within the meaning of section 5(b) of the War Powers Resolution.
(2) APPLICABILITY OF OTHER REQUIREMENTS- Nothing in this joint resolution supersedes any requirement of the War Powers Resolution.
SEC. 4. REPORTS TO CONGRESS. (a) REPORTS- The President shall, at least once every 60 days, submit to the Congress a report on matters relevant to this joint resolution, including actions taken pursuant to the exercise of authority granted in section 3 and the status of planning for efforts that are expected to be required after such actions are completed, including those actions described in section 7 of the Iraq Liberation Act of 1998 (Public Law 105-338).
(b) SINGLE CONSOLIDATED REPORT- To the extent that the submission of any report described in subsection (a) coincides with the submission of any other report on matters relevant to this joint resolution otherwise required to be submitted to Congress pursuant to the reporting requirements of the War Powers Resolution (Public Law 93-148), all such reports may be submitted as a single consolidated report to the Congress.
(c) RULE OF CONSTRUCTION- To the extent that the information required by section 3 of the Authorization for Use of Military Force Against Iraq Resolution (Public Law 102-1) is included in the report required by this section, such report shall be considered as meeting the requirements of section 3 of such resolution.
Since the U.S. didn't return to the U.N. for a final resolution autorizing the invasion, and by U.S. law/treaty the we are bound to the U.N. process it is again, at a minimum, debatable, rather than a matter of established fact upon which one should be called a liar.