Having warned Syria’s Bashar al-Assad in August 2012 that the use of chemical weapons on his own people was a “red line” that would compel the United States to respond with force, President Barack Obama now apparently feels the need to follow through on his threat. In preparation, his diplomats are seeking international backing, albeit with limited success. The Arab League did not condone a US military response, understandable given the views of its member countries’ populations about US military actions generally. On Wednesday, UN envoy Lakdar Brahimi said “international law is clear” that any such military action requires approval by the UN Security Council, but Security Council approval is hostage to the certain vetoes of China and Russia.

But while Obama sends his representatives around the world to obtain support and gain more legitimacy for a US-led military response, he has not sought the approval of the one body whose authority is clearly required: the United States Congress. A military intervention in Syria, even if it was confined to an unmanned cruise missile attack, would unquestionably constitute an act of war. And whatever the validity of such an act under international law, under the US Constitution only Congress has the authority to authorize war or lesser forms of military force. The Framers gave that power to the legislature precisely because they wanted to make going to war more difficult, and knew that presidents would be more inclined to bellicosity than a multi-member legislature.

Presidents have of course often used military force without Congress’s prior approval, although they have generally attempted to fit their actions within the long-recognized exceptions to the principle of Congressional authorization: namely, when they must act in self-defense to an armed attack or imminent armed attack, leaving no time for recourse to Congress. But there is no conceivable justification along these lines for a military strike on Syria. President Obama is responding to an attack allegedly ordered and carried out by the Assad regime on its own citizens, not on the United States or American citizens.

Some have argued that the 1973 War Powers Resolution authorizes the president to undertake unilateral short–term military interventions, because one provision of that law requires presidents to report to Congress upon the introduction of troops into hostilities or imminent hostilities, and establishes a sixty-day clock for obtaining congressional approval thereafter. But Congress was very explicit that this mechanism, designed to rein in presidential war making, was not intended to give presidents a two-month grace period to make war as they see fit. As Congress expressly provided:

Nothing in this chapter … shall be construed as granting any authority to the President with respect to the introduction of United States Armed Forces into hostilities or into situations wherein involvement in hostilities is clearly indicated by the circumstances which authority he would not have had in the absence of this chapter.

Obama himself acknowledged the point in an interview with The New York Times during his first presidential campaign, stating:

The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.

On Wednesday, House Speaker John Boehner wrote the president a letter raising numerous questions about the proposed action, and specifically requesting Obama to explain “on what basis any use of force would be legally justified and how the justification comports with the exclusive authority of Congressional authorization under Article I of the Constitution.” Earlier this week, Republican lawmaker Scott Rigell, who represents a Congressional district in Virginia with one of the highest numbers of active-duty US servicemen and women in the country, also asked the president to allow Congress to debate any proposed military intervention.

And a bipartisan group of former members of Congress and legal scholars, part of the War Powers Committee of the Constitution Project, sent a letter to the president earlier this summer, similarly stressing the constitutional requirement of congressional authorization. To date, President Obama has not responded.

It is possible that the military action now being contemplated by the White House might qualify as “humanitarian intervention,” on the ground that it is designed to forestall further atrocities in Syria. Whether such a response in the absence of UN Security Council approval is permissible under international law is a matter of debate, although most legal scholars would argue that Security Council approval is required. But again, under our Constitution, there is no exception to the requirement of Congressional approval for humanitarian interventions. Any hostile use of military force in another sovereign’s territory without its consent is an act of war, and requires Congress’s assent.

If President Obama ignores this requirement, he won’t be the first. President Clinton ignored it when he gave NATO authorization to use US forces to bomb Kosovo. President Reagan did so when he supported the contras in Nicaragua’s civil war, and when he sent troops to Grenada. President Truman did so in Korea, which he called a “police action,” fooling no one. In fact, the reason Congress enacted the War Powers Resolution was that presidents had too often seized the advantage by unilaterally introducing troops, and only then, if at all, coming to Congress for authorization after the fact, when Congress had no real choice but to support the president.

The decision to intervene in Syria is, by all accounts, a difficult one. It is far from clear that it will do any good, and there is substantial risk that it will do harm. Polls show many Americans are against intervention. Iran has threatened to respond by attacking Israel. Assad may well have concluded that drawing in US forces will make him more popular, not less so, given attitudes toward the US in his country and the region. And it’s not clear that the forces currently fighting Assad would offer a less violent future for Syria if he is removed. President Obama, recognizing the risk of going it alone, has dispatched his emissaries to drum up support around the globe. But in view of the many risks involved, he would do well to seek support from Congress as well. That’s the red line the Framers drew, and the president should respect it.

Update: On August 31, the president changed his position on a military strike on Syria, announcing that he would seek authorization from Congress.