The Volokh Conspiracy
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A cross-ideological group of constitutional and national security law scholars recently submitted a letter to Speaker of the House Nancy Pelosi urging her and the House of Representatives to initiate a lawsuit to halt the illegal US role supporting Saudi Arabia and its allies in the war in Yemen. The letter was drafted by Yale Law School Professor Bruce Ackerman, with assistance from the other participants. Signatories include Ackerman, Richard Albert (University of Texas), Rosa Brooks (Georgetown), Erwin Chemerinsky (dean of the law school at UC Berkeley), Mary Dudziak (Emory), Michael Glennon (Tufts), Jon Michaels (UCLA), Mary Ellen O'Connell (Notre Dame), Michael Ramsey (Univ. of San Diego, and one of the authors of the Originalism Blog), Aziz Rana (Cornell), Scott Shapiro (Yale), Ruti Teitel (New York Law School), and myself, among others (institutional affiliations listed for identification purposes only).
Here is an excerpt:
In vetoing Congress' joint resolution on Yemen, President Trump has defied fundamental principles of constitutional law laid down by the Supreme Court's landmark decision in the  Steel Seizure Case. The Court's decision involved a genuine emergency. A steelworkers' strike had halted production, and this led to a dramatic reduction of crucial war materiel required by American troops fighting in Korea. Faced with a clear and present danger to the war effort, President Truman seized the steel mills in his capacity as Commander-in-Chief and ordered the workers back to work. In taking this step, Truman refused to follow specific provisions of the Taft-Hartley Act that Congress had laid out to deal with strikes in national emergencies. He instead declared that, as Commander-in-Chief, he had the power to act independently of the law laid down by Congress. The Supreme Court rejected Truman's assertion of unilateral power as unconstitutional in the Steel Seizure Case…
We call upon you, as Speaker of the House, to initiate a law-suit which calls upon the judiciary to vindicate Steel Seizure in the case of President Trump's military support of the Saudi war against Yemen. President Trump raises the very same constitutional question decided by Youngstown – only this time, it is the War Powers Resolution, not the Taft-Hartley Act, which explicitly prohibits the president from using his power as commander-in-chief to engage in unilateral war-making.
President Trump's decision to support the war in Yemen represents a clear violation of the  War Power Resolution's reaffirmation of the Founder's grant to Congress over the ultimate question of war and peace. Section 8(a)(c) not only grants Congress power to forbid American troops from engaging in "hostilities" involving direct acts of violence. It explicitly defines "hostilities" very broadly to enable the House and Senate to prohibit American armed forces from engaging in actions which "coordinate" or "accompany" the "regular or irregular military forces of any foreign country." Congress was acting well within its constitutional authority in insisting on this broad definition of "hostilities." Given the ease with which military "coordination" with foreign powers can escalate into full-blown war under modern conditions, the Constitution's "necessary and proper" clause gave Congress ample authority to include these indirect forms of military support in order to preserve its ultimate authority "to declare war."
I offered some additional analysis of the illegality of US intervention in the Yemen War here (in a post that reflects solely my own views, and not necessarily those of other signers of the letter):
But Trump's veto of the resolution is not enough to make the US role in this conflict legal. It is still in violation of the 1973 War Powers Resolution… That legislation forbids the "introduction" of US forces into "into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances," for a period of more than 90 days without congressional authorization…. Significantly, the WPR defines "introduction" into hostilities to include "the assignment of member[s] of [the US] armed forces to command, coordinate, participate in the movement of, or accompany the regular or irregular military forces of any foreign country or government when such military forces are engaged, or there exists an imminent threat that such forces will become engaged, in hostilities."
While US forces are not directly engaged in combat in Yemen, the Trump Administration itself admits that they have provided intelligence, logistical support, and—at times—even in-flight refueling of Saudi aircraft. As Utah Republican Sen. Mike Lee…, puts it, "We're literally telling the Saudis what to bomb, what to hit, and what and who to take out." That pretty clearly amounts to US involvement in the command, coordination, and "movement" of Saudi forces—exactly the sort of thing that the WPR forbids, absent congressional authorization.
US involvement in the Yemen War dates back to the Obama administration, and has long since passed the 90 day WPR deadline. Congress has never voted to authorize that involvement. Thus, it is illegal.
There is widespread bipartisan concern in Congress about the illegality of this conflict and the very dangerous precedent it sets. That is why the Yemen resolution passed in the first place, with support ranging from conservatives such as Senator Lee, to libertarians like Justin Amash, and virtually all Democrats. Rep. Ro Khanna, Vice-Chair of the House Progressive Caucus likewise supported the Yemen resolution, and now has also endorsed the lawsuit plan.
Unfortunately, widespread concern cannot stop the intervention by traditional legislative means alone, because the president can veto any congressional resolution he opposes, and the veto can only be overriden by an overwhelming two-thirds majority in both houses of Congress. By circumventing the War Powers Act Trump (like Obama before him) has shifted authority away from Congress to himself, ensuring that the default position is that he can continue the intervention, rather than that he must stop it unless Congress gives its affirmative consent. That makes a hash of the Founding Fathers' scheme to ensure that the president cannot enter into new international conflicts without getting advance congressional approval.
A lawsuit could help redress this imbalance by enabling Congress to stop the illegal legislation without having to through a process in which the president can use the veto to shield his power grab. Even though success is far from certain, it is a strategy that deserves to be tried.
Some will likely condemn this strategy because the courts may dismiss such a lawsuit for lack of "standing." That could happen. But, in my view, Congress has a strong basis for standing in cases where the president has appropriated a core congressional power for himself.
In order to get standing to sue, a plaintiff, must prove that it has 1) suffered an "injury in fact" that is "concrete" and "particularized," 2) that there is a causal link between the injury and the defendant's supposedly illegal conduct, and 3) that the injury can be redressed by a judicial ruling. Presidential circumvention of the War Powers Act inflicts a "concrete" and "particularized" injury on Congress by depriving it of its share of control over the deployment of US military forces—an extremely important national asset. In addition, there is no doubt there is a causal link between the president's actions and Congress' injury. And a court can redress the injury by ordering a halt to unauthorized US military assistance to the Saudis and their allies.
Presidential usurpation of congressional war powers is not a new problem. Along with others, Bruce Ackerman and I spoke out against it during the Obama years. The time has come to consider new strategies for reining in the executive in order to ensure that no one person has the power to take the nation to war. As James Madison put it, "[i]n no part of the constitution is more wisdom to be found, than in the clause which confides the question of war or peace to the legislature, and not to the executive department…. [T]he trust and the temptation would be too great for any one man…"