It was a resounding defeat for the White House and Republican Senate leadership: by a 63-37 majority, the US Senate voted this week to advance legislation that would give President Trump 30 days to get the US military out of Saudi Arabia’s genocidal war in Yemen, unless he could get congressional authorization for US military intervention. Which he almost certainly could not.

The vote on Wednesday was procedural, allowing the legislation spearheaded by Bernie Sanders, Mike Lee and Chris Murphy – to move toward a full Senate vote. But it is widely seen as a reliable indicator of what a Senate vote on the resolution itself would look like.

The Senate’s action was truly historic for a number of reasons. First there is the magnitude of the war crimes that the Senate is trying to end. Mass starvation has been used as a weapon of war by the Saudis and their Emirati allies, pushing 14 million people to the brink of famine. More than 85,000 children have already died since their bombing campaign began in 2015. As was noted during the Senate debate on Thursday, the Saudi and UAE planes have also bombed water treatment plants and other essential civilian infrastructure, leading to a cholera outbreak that has killed thousands of people.

Democratic Senators such as Murphy, Lee, and Dick Durbin of Illinois were unusually frank in the Senate debate. They used words like “involvement” and “participation” to describe the US military role – in contrast to the fuzzier descriptions of “support” and “complicity” that are often seen in the media.

In a recent interview Senator Murphy said:

I think there is an American imprint on every single civilian death inside Yemen. We sell them the bombs, we help them with the targeting, we fuel their planes in mid-air, and we give them moral cover. So I don’t think there is any way around complete American culpability for the humanitarian nightmare that is happening there.

In addition to the public educational benefits of this kind of honesty, it’s important as a legal matter because of another historic, path-breaking feature of this senate action. The vote represents the first time that the US Senate has invoked the 1973 War Powers Resolution in an effort to end unauthorized US military participation in a war.

The War Powers Resolution states explicitly that the president can only introduce US armed forces into a situation of hostilities or imminent hostilities if there is congressional authorization (with an exception for some responses to attacks on the United States, which no one is claiming would apply in this war).

This reiterates what is provided by article I of the US constitution, as a number of senators pointed out in Wednesday’s debate. The law also spells out the means by which Congress can assert its constitutional authority to end unauthorized US military participation in a foreign conflict, which is precisely what the Congress is doing.

The historic significance of restoring the constitutional power of Congress on matters of war and peace was not lost on the Trump administration, its allies, and all who believe that war is a presidential prerogative. They lobbied hard to defeat this resolution. US secretary of state Mike Pompeo and defense secretary James Mattis held a classified briefing with senators before the vote, and the White House threatened to veto any such legislation if it passed. But 14 Republicans joined all 47 Democrats to rebuke them.

While it is true that the Saudis’ brutal murder of Washington Post journalist Jamal Khashoggi has influenced the political debate on this war, this resolution was close to passing in March, before the assassination. And as the lopsided margin indicates, it would almost certainly have passed in any case. Grassroots efforts by dozens of organizations across the country clearly played a significant role – most of the 10 Democrats who switched their position since the last vote didn’t make this decision known until the 24 hours before the vote.

The increasing publicity given to the war crimes in Yemen, their sheer scale and now the undeniability of US military participation may have forced some US politicians on the fence to rethink their positions. Do they want to become known and remembered for voting to kill millions of innocent people in Yemen?

What happens next? Trump wants people to think that this effort can’t go anywhere because he will veto any legislation to cut off US military involvement. But the War Powers Resolution provides for a procedure under which a simple majority of both chambers can be veto-proof, instead of the usual requirement for a two-thirds majority. Both the Senate and the House would have to pass concurrent resolutions demanding withdrawal; this bicameral directive to end unconstitutional activities could not be vetoed by the president.

This is the path that Congress is on right now. In the House, Ro Khanna, Mark Pocan, and Jim McGovern vowed to “follow suit” in the wake of a successful Senate vote “by promptly passing a similar War Powers Resolution in the House.” Republican leadership derailed their previous bipartisan effort two weeks ago through procedural skullduggery while dozens of Representatives were not even back in town from recess.

If they can’t win in the House before the new Congress is seated in January, they will certainly prevail afterward; their resolution is already co-sponsored by the Democratic leadership, including Nancy Pelosi and the incoming Democratic chairs of the most important House committees. Then the Senate can pass a companion concurrent resolution to force the president to withdraw, and follow up through defunding offensive US activities in Yemen.

In an op-ed published by the Wall Street Journal the morning of the vote, Pompeo showcased his indifference to the atrocities of his Saudi allies. “The US doesn’t condone the Khashoggi killing,” he wrote, in what was possibly the faintest disapproval ever of the cold-blooded murder of a journalist. He then dismissed the international outcry over the assassination as “caterwauling” – the shrieking of cats.

This is what we are dealing with. No wonder he didn’t win over any senators in his briefing before the vote. In the coming weeks, Congress will force the US military to withdraw from Yemen. Given the magnitude of the humanitarian crisis, there’s not a moment to spare.