No doubt about it, most of the wars the US is currently fighting are unconstitutional. From Danny Sjursen at truthdig.com:

[T]he President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons. …
—S.J. Res. 23 (107th): Authorization for the Use of Military Force (AUMF), Sept. 18, 2001
The President is authorized to use the Armed Forces of the United States as he determines to be necessary … in order to … defend the national security of the United States against the continuing threat posed by Iraq. …
—H. J. Res 114 (107th): Authorization for the Use of Military Force Against Iraq, Oct. 18, 2002
It’s all so obvious to a detached observer. Nonetheless, it remains unspoken. The United States of America is waging several wars with dubious legal sanction in domestic or international law.
The U.S. military stands astride the Greater Mideast region on behalf of an increasingly rogue-like regime in Washington, D.C. Worse still, this isn’t a Donald Trump problem, per se. No, three successive administrations—Democratic and Republican—have widened the scope of a global “war” on a tactic (terror), on the basis of two at best vague, and at worst extralegal, congressional authorizations for the use of force (AUMF). Indeed, the U.S. is veritably addicted to waging undeclared, unwinnable wars with unconvincing legal sanction.
Despite 17 years of fighting, dying and killing, there have been no specific declarations of war. Instead, one president after another, and hundreds of derelict-in-their-duty congress members, have simply decided on their own that a vague resolution, rubber-stamped while the rubble in New York was still smoking, authorizes each and every conflict in which America’s soldiers—and many more civilians—continue to die. This AUMF authorized the president to kill or capture those who perpetrated the 9/11 attacks, but, well, few of America’s current adversaries had anything to do with that.
To continue reading: Illegal Wars: The New American Way