May the U.N., rather than the elected representatives of Congress, authorize the United States to use military force against another nation? Is it possible to transfer the constitutional power of Congress to an international body? The answer to both questions: No. Authority under law and the Constitution must come from Congress. Statutory law, dating to 1945, speaks unambiguously about the use of American troops in a U.N. military operation: "The President is authorized to negotiate a special agreement or agreements with the Security Council which shall be subject to the approval of the Congress by appropriate Act or joint resolution." 22 U.S.C. 287d.Historical practice since 1945, he argues, "shows that members of Congress have failed to protect the Constitution, their own institutional powers and the rights of citizens who elected them to office." The rest is here.
On the Lawfare blog, Curtis Bradley also takes up "the role of historical practice in debates over separation of powers, including in debates over the Constitution’s distribution of war authority."
There are two basic positions on the constitutional distribution of war powers – a pro-Congress view and a pro-Executive view. Although it has some variants, the pro-Congress view generally contends that the President is constitutionally required to obtain congressional authorization for the use of military force except when defending the United States from attack or protecting the safety of U.S. citizens abroad. The pro-Executive view, by contrast, contends that the President may use military force abroad without congressional authorization, and that Congress’s ability to check the President stems primarily from its control over appropriations.
The military operations in Libya have once again triggered a substantial debate over war powers. The positions in this debate are illustrated by, on the one hand, an opinion from the Office of Legal Counsel, and, on the other hand, an essay critiquing the opinion by Professor Michael Glennon. As is evident from these materials, debates over war powers are often in part debates over the interpretation and weight to be given to historical practice — in particular, the instances in which presidents have initiated the use of military force without congressional authorization. The OLC opinion on Libya, for example, recites various instances of “such presidential initiative,” whereas Glennon argues that many of these instances are distinguishable.
In thinking about the Libya debate, I’ve started to have greater doubts, or at least uncertainties, about the proper role of historical practice in constitutional interpretation relating to the separation of powers.
(emphasis added) Ultimately, Bradley suggests that "constitutional claims based on practice are made too readily in a variety of contexts and that we need to think more deeply about when and why this practice should matter." The full post is here, and another is here.