SAUDI ARABIA’S war in Yemen is a humanitarian disaster. Airstrikes, cholera and hunger have killed tens of thousands of people. America is an integral, if backstage, party to the conflict. It supplies planes (serviced by American mechanics), weapons and intelligence to the Saudis. American lawmakers—so far inert—have finally woken up to the war’s toll. The murder of Jamal Khashoggi, a Saudi journalist, by Saudi goons and President Donald Trump’s feeble response to it have shifted the political calculus. Now Congress is poised to demand an end to military support for the Saudi campaign: the Senate voted to do so on March 13th; the House of Representatives will follow suit soon. Mr Trump will probably veto the resolution if it crosses his desk. Both sides say they are exercising their constitutional prerogative. So, who authorises military action—Congress or the president?

The constitution gives the power to Congress. The framers, wary of concentrated authority and of replicating a monarchy in America, empowered the legislature to raise armies and declare war. The president would lead the military as commander-in-chief and defend the country against attack. But Congress was to initiate any offensive action. James Madison warned of an overreaching, warmongering president in 1793 when he called war “the true nurse of executive aggrandisement”. Delegating responsibility this way worked as intended until the mid-20th century. Then Harry Truman unilaterally deployed troops in the Korean War (pictured), calling it a “police action” and setting a precedent for presidents to bypass Congress. A decade later, Lyndon Johnson secured congressional authorisation for American involvement in Vietnam, promising that it would be “limited and fitting”; it lasted ten more years and expanded furtively to Cambodia and Laos. This infuriated lawmakers. Keen to reassert authority, Congress passed the War Powers Resolution in 1973. It requires presidents to report troop deployments within 48 hours and seek authorisation within 60 days or withdraw.

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But presidents defy it; they think it unconstitutional given their role as commander-in-chief. They have done so repeatedly, in Grenada, Somalia, Kosovo, Haiti, Libya and elsewhere. (For major deployments—Iraq in 1991 and 2002, for example—they have sought authorisation, confident they will get it.) The effect has been to reinforce executive power in matters of war. The reality is that a president holds certain advantages over Congress. He can act alone and decisively, and thus set the terms of debate. He has intelligence that Congress cannot easily match. But lawmakers aren’t impotent. They can cut funding for military ventures and specify how money is spent. Through hearings they can raise the political costs of failure. The expectation of congressional meddling affects presidents’ behaviour. William Howell and Jon Pevehouse, two political scientists, have found that presidents opt for force less often and less quickly when the opposition party holds the majority in Congress.

Action from Congress over Yemen is a rebuke that Mr Trump will ignore. (He reasons that American forces are not in the line of fire and so are not engaged in “hostilities” as defined by the War Powers Act.) Lawmakers could still halt arms sales and intelligence-sharing, though Mr Trump would veto those measures, too. Expect no tidy resolution to a devastating conflict.