Even as the debate over a possible US confrontation with Iran continues, the US continues to support the Saudi-led war against Iranian-backed Houthi rebels in Yemen. Last month, President Donald Trump vetoed a congressional resolution that would have terminated US military aid to Saudi Arabia and its allies in the Yemen conflict; the resolution was backed by virtually all congressional Democrats, as well as seven Republican senators and sixteen GOP members of the House.

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 (also known as the War Powers Act). 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 (an initial 60 day period, followed by an additional 30 day extension). 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 (a co-sponsor of the Yemen resolution), 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.

Trump's veto of the recent Yemen Resolution does not change that. That Resolution was an exercise of Congress' authority under Section 5(c) of the WPR, which allows Congress to use a "concurrent resolution" to terminate a conflict "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. This enables Congress to end US involvement even if the 90 day WPR deadline has not yet been exceeded. But that deadline still applies to cases where Congress has not passed a concurrent resolution terminating US involvement or the president has vetoed such a resolution (as in the Yemen case). The Trump administration, like the Obama administration before it, remains in violation of the WPR.

At least for the time being, US involvement in the Yemen conflict remains very limited. But that could change over time, especially if the war drags on and the Saudi-led coalition continues to fail to achieve its objectives. In that event, the US could become more seriously enmeshed in the conflict.

This case is by no means the first recent illegal presidential use of war powers. The Obama administration, for example, started an illegal military intervention in Libya, which violated both the War Powers Act and the Constitution (which reserves the power to initiate war to Congress). Obama also started—and Trump continued—the war against ISIS without proper congressional authorization. But the fact that illegal US involvement in the Yemen conflict is not a unique case doesn't make it right. To the contrary, the ongoing nature of the problem makes it all the more important for Congress to reassert its control over the initiation of war. The Yemen resolution was a step in the right direction. But Congress can and should do much more.

The issue is more than just a matter of technical legality, of interest only to legal scholars. Legislative control over the initiation of war prevents that decision from falling under the control of any one person, and ensures that we only enter a conflict if there is a broad political consensus in favor of doing so. Along with others, I did what I could to to make that case during the Obama years, and it remains just as valid today.

Meanwhile the Yemen War continues, having already killed an estimated 67,000 people, and created millions of refugees. The principal blame for all that death and suffering rests with the combatants. Both sides have committed their share of atrocities. But US support for the Saudi-led coalition has helped make the situation even worse than it might be otherwise. And we have little, if any, benefit to show for it. Certainly none that even comes close to justifying the enormous human cost of the war. That is morally problematic in itself, even aside from the possibility that the US could become more deeply involved in the war.

There is plenty of room for reasonable disagreement over how best to deal with the danger posed by Iran. I am very skeptical that all-out war is the right approach. But I am also somewhat more hawkish than many other libertarians, and more open to various types of military action than many of them.

But regardless of the specific policy in question, we should be able to agree that the US should not initiate war without proper legislative authorization. For the moment, President Trump seems to have overruled his more hawkish advisers and tried to pull back from military confrontation. But that decision should not rest in the hands of the president alone. As James Madison put it, "the trust and the temptation would be too great for any one man."