Bernie Sanders wants to talk about Yemen.

“I don’t think people fully appreciate the significance of foreign policy,” Sanders griped to me in his Senate office in late April, after two straight weeks on the campaign trail. The media, he said, doesn’t cover the world enough.

Sanders was criticized on this exact point — for lacking the experience, let alone interest, in matters abroad — when he was running against former Secretary of State Hillary Clinton. He doesn’t dispute it; “you evolve,” he told me. Now he is meticulous about weaving in foreign policy on the campaign stump or at a presidential town hall, whether or not it’s asked of him.

Specifically, he focuses on Yemen. For four years, the United States has been helping the Saudis wage a bloody war against Iranian-backed rebels there, giving arms, ammunition, intelligence, and more. The war, among the worst humanitarian crises in the world, has killed more than 50,000 people, according to one independent estimate, and has left tens of millions more in need of assistance. Sanders signed on to a historic movement in Congress to end US involvement two years ago — a movement that caught steam when it became clear that Saudi Crown Prince Mohammed bin Salman, or MBS, had called for Saudi Washington Post journalist Jamal Khashoggi’s killing.

It took every ounce of political will fighting Saudi Arabia’s powerful lobby, a legacy of hawkish anti-Iran lawmakers — both Democratic and Republican — and President Trump’s personal affinity for MBS to pass the War Powers Resolution through the Republican-controlled Senate and the Democrat-run House, where it was led by Rep. Ro Khanna (D-CA) (who is, incidentally, now Sanders’s presidential campaign co-chair). It was the first time Congress has ever adopted such a resolution, directing Trump to remove troops involved in a war Congress never authorized. When it passed, Sanders went on Fox News and urged Trump to sign it. The next day, Trump vetoed the resolution.

Even so, for Sanders, the War Powers Resolution has served as a clarifying issue for his 2020 foreign policy message. The fight against the Yemen war fits so perfectly within his worldview that to listen to him explain it, you can hear the echoes of his famed speeches about millionaires and billionaires on Wall Street.

“MBS, who I think has the unique distinction of owning the largest home in the world and the largest yacht in the world — he should not be determining American military and foreign policy,” Sanders told me with a smile.

At first, “there was a fear of being the next Dennis Kucinich”

This story begins in 2015 with an antiwar activist no one would listen to: Robert Naiman, the director at the progressive group Just Foreign Policy, who saw what was happening in Yemen.

“The war was horrible immediately; the war crimes started immediately,” said Naiman, who thought of the idea for Congress to invoke the War Powers Act of 1973 and pass a resolution that would direct the president to remove troops involved in “hostilities” abroad “without a declaration of war or specific statutory authorization.”

Congress established this power — over Richard Nixon’s veto — to prevent another Vietnam War. A War Powers Resolution (WPR) is first and foremost Congress clawing back its constitutional powers from the president; it is an assertion that the executive has overstepped their bounds as commander in chief, going to war without Congress’s approval. It’s particularly poignant today because for the past 18 years, presidents of both parties have used the same 2001 congressional war authorization — passed after 9/11 — as justification for wars all over the Middle East. Yet Congress has never successfully invoked a WPR. Naiman’s idea was to do something historic and pass a WPR on Yemen for the first time since the power was created in 1973.

The Obama administration said getting involved in Yemen wasn’t planned. They were in the middle of tense negotiations both domestically and with international allies to get the Joint Comprehensive Plan of Action — the Iran deal — over the finish line when a coalition led by Saudi Arabia and its allies began a military campaign in Yemen against Iranian-backed Houthi rebels. Since then, the United States began providing the Saudis intelligence, arms and ammunition, and, until late last year, fuel for their warplanes. The warplanes that bombed a school bus full of children, killing at least 40 of them last August, did so with an American-made bomb.

“Because Obama started [US involvement in the] war, many Democrats were reluctant to challenge it,” Naiman said. “[Obama’s national security advisers] Ben Rhodes, Jake Sullivan, all these people had their fingerprints on their policy, and they knew what was happening in Yemen.”

In the House, Naiman tried to get Rep. Ted Lieu (D-CA), who early on had raised concerns about civilian deaths in Yemen, to lead the push for the resolution, but his office turned it down. Sen. Chris Murphy’s (D-CT) office also decided against it. Murphy had been one of the first lawmakers outspoken about Yemen; he went on CNN and said the United States had “made a decision to go to war” without congressional authorization. In emails reviewed by Vox, Murphy’s staff, asked what gave the U.S. government the authority to bomb the Houthis in Yemen, said the U.S. wasn’t conducting the bombing of the rebels — just selling the bombs and the intelligence — and that the Administration could conceivably justify the strikes against Al Qaeda under the 2001 authorization. They concluded a war powers resolution wasn’t the correct strategy at the time, and pursued a bipartisan resolution to block arms sales to Saudi Arabia with Sen. Rand Paul (R-KY).

Two Democrats — one lawmaker, one congressional aide — described Democrats’ reluctance to get behind the WPR the exact same way.

“There was a fear of being the next Dennis Kucinich,” they said, invoking the former Ohio lawmaker, whose strong anti-interventionist record may be in today’s zeitgeist but who has been pegged as a wacky outsider throughout his career. A WPR was seen as radical. It had never been done before. And to pursue it at the time meant not only questioning Obama’s strategy with Iran but interfering with the longstanding alliance with Saudi Arabia.

Then Naiman found Rep. Ro Khanna (D-CA): an anti-Iraq War progressive who came into office in 2017. He was the grandson of a human rights activist and was an Obama organizer in 2008 but had never been a politician under Obama — and he would go on to endorse Bernie Sanders in 2016 and be his campaign surrogate in 2020.

The Yemen War Powers Resolution captured a sea change in the Democratic foreign policy

Khanna decided to endorse Sanders in 2016 after seeing the senator debate foreign policy on Chris Matthews’s Hardball.

“Chris Matthews and he were talking about the overthrow of Patrice Lumumba in the [Democratic Republic of] Congo,” Khanna said. “Belgian colonialism was arguably the most cruel — crueler even than what my grandfather faced under the British. I thought if someone understands the history of Patrice Lumumba, they will stand up for human rights and a more just foreign policy.

“My instinct was vindicated when Sen. Sanders became the only one in the Senate willing to take a risk in introducing the war powers resolution to end the war in Yemen.”

Sanders joined the movement to end US involvement in Yemen in November 2017. By that point, Khanna had built a bipartisan coalition in the House of progressives like Rep. Mark Pocan (D-WI) and constitutional conservatives like Reps. Thomas Massie (R-KY) and Walter Jones (R-NC), angry about another war without congressional authorization.

Over the course of two years, support for the Yemen resolution became a litmus test of the Democratic Party’s progressive shift on foreign policy. Obama’s national security advisers, like Ben Rhodes, Jake Sullivan, and Robert Malley, and former US Ambassador to the United Nations Samantha Power called the decision to get the United States involved in Yemen a mistake.

Photographs of starving children and American bombs killing civilians became hard to ignore; each iteration of the atrocities brought in more Democratic support. Then in October 2018, Jamal Khashoggi, the Saudi dissident journalist with the Washington Post, was killed and allegedly dismembered inside Saudi Arabia’s consulate in Istanbul. Lawmakers, many of whom had historically backed the US-Saudi relationship, became incensed by MBS’s attempted deception about what had happened and the Saudis’ perceived assumption that the United States would remain idle. Even Sen. Lindsey Graham (R-SC), a staunch Trump ally, said he felt “completely betrayed” by Riyadh.

Sanders led the effort in the Senate, with Murphy and Utah Republican Sen. Mike Lee by his side. The coalition was based on three principles: 1) that it’s Congress’s constitutional right to declare war, not the president’s; 2) that Yemen was an unbelievable humanitarian disaster; and 3) that Saudi Arabia shouldn’t be dictating American foreign policy.

Still, “it was deeply unpopular with leadership in both parties,” Khanna said of the early effort. “There were concerns about whether we needed the Saudis as a counter to Iran. We didn’t have the support at the time of even our own party.”

To list the times the momentum around the War Powers Resolution on Yemen was brought to a crashing halt is exhausting. There was the time a House bill about controlling the American wolf population included a provision banning debate on the Yemen WPR. The farm bill included a provision that banned a vote on the Yemen resolution in 2018. Republicans won Democratic votes for an amendment to ensure the resolution would not “disrupt ... the sharing of intelligence between the United States and any foreign country if the President determines such sharing is appropriate,” essentially undermining the purpose of the resolution altogether. With each change, the WPR would volley between the House and Senate, unable to pass.

Top Trump administration officials, and Saudi lobbyists who are well established in Washington, worked hard to build support for continued US involvement in the Yemen war. Then-Defense Secretary Jim Mattis claimed that stopping US support “could increase civilian casualties, jeopardize cooperation with our partners on counterterrorism, and reduce our influence with the Saudis — all of which would further exacerbate the situation and humanitarian crisis.” However, last October Mattis also pushed for a negotiated peace deal to end the fighting — a peace deal that remains elusive.

Among Democrats, there was still confusion about how much involvement the United States had in Yemen. It took aggressive campaigning on the part of antiwar activists to get lawmakers on board. It took Democrats winning back the House majority, Speaker Nancy Pelosi prioritizing a vote, and Majority Leader Steny Hoyer to defend the resolution against bad-faith Republican amendments.

Through all this, Trump, having waffled in his response to the killing, repeatedly emphasized his support for MBS, calling him a “great ally.” In March, reports showed that the United States approved six secret authorizations to sell Saudi Arabia nuclear power technology, which both Democratic and Republicans lawmakers have warned could aid a nuclear arms race in the Middle East.

But MBS had shown his “true colors,” Rhodes said, and the momentum carried the WPR through the Senate and House on April 4, 2019. It was the first time since the act was passed in 1973 that both chambers of Congress sent a WPR to the president’s desk. Trump vetoed it.

A Bernie Sanders-style political revolution in foreign policy

The Yemen WPR officially died on May 2, 2019, when all but five Republican senators refused to override Trump’s veto. In the days leading up to that final vote, Sanders’s staff was distributing the most recent United Nations report on Yemen to every Senate office. They wanted every lawmaker to know that 80 percent of the Yemeni population — 24 million people — is in need of humanitarian assistance, living through bloody warfare and widespread famine. It didn’t change enough minds.

But every single Democrat in the Senate voted in favor of the War Powers Resolution that day, something almost unimaginable three years ago. Since then, a group of constitutional scholars has sent Pelosi a letter urging her to bring a lawsuit against Trump for vetoing the resolution.

“It’s no small achievement to essentially unite the entire Democratic caucus in both houses behind this progressive position and also to peel off a little bit of Republican support,” Rhodes said. “It shows there is actually progressive momentum in the Democratic Party that wasn’t there when Obama was in office. We had to work harder to get votes for the Iran deal.”

And the resolution, even though it failed, influenced other world actors.

“We all want member states to think carefully, and if the US doesn’t, we will be the poorer,” Martin Griffiths, the UN envoy to Yemen, told me, lauding the debate around the Yemen resolution but unable to comment further on American politics.

“The message conveyed by the War Powers Resolution has been heard in capitals that matter, regardless of president Trump’s veto,” Robert Malley, who served on Obama’s National Security Council, said. “In Riyadh, as in Abu Dhabi, it has an impact because leaders of those countries can read our politics and can see where the bipartisan winds are blowing.”

Sanders said he saw “an opportunity to bring together principled people from the right and the left.” But it was also an opportunity to translate his revolution to matters abroad. This time, his fight wasn’t against the CEOs in Silicon Valley or hedge fund managers on Wall Street; he’s talking about the oligarchs of the world, and the powerful people who prop them up.

For months leading up to his 2020 bid, Sanders has been laying out his vision for a progressive foreign policy. It’s one that sits directly in concert with his view of American politics. “We need an international movement that mobilizes behind a vision of shared prosperity, security, and dignity for all people,” he said at a December speech at Johns Hopkins University. As Vox’s Alex Ward put it, his thesis is that “income inequality and authoritarianism are intricately linked.”

In his Senate office, he explained it to me in these terms: “Rather than saying ‘we love Saudi Arabia’ — I don’t — and ‘we hate Iran,’ let’s bring people together. Iran has its many faults as well. I’m not here to be praising Iran. But the United States should be playing a role in bringing people together and not just supporting a murderous dictatorship.”

Sanders’s leadership on Yemen could truly set him apart in a crowded field

On the presidential campaign trail, only two candidates have meaningfully occupied this progressive space on foreign policy: Sanders and Sen. Elizabeth Warren (D-MA).

They’ve called for pulling troops out of Afghanistan and Syria. They’ve rebuked Israel’s right-wing leadership. And, of course, they’ve been outspoken on Saudi Arabia and the war in Yemen. The call for a more “values-based” foreign policy rests on a simple principle: that “every person on this planet shares a common humanity,” Sanders said at a foreign policy speech at Westminster College.

The winning politics of this kind of messaging is clear. Obama, Sanders, and, to a large extent, Trump — who attacked Hillary Clinton for supporting the Iraq War — all ran as anti-interventionist candidates. US involvement in Yemen sits perfectly at odds with this worldview, at the nexus of an unrelenting war abroad and a corrupt foreign alliance.

“On a lot of issues, Trump is further to the left on foreign policy than a lot of the [Democratic presidential] candidates, [on] North Korea, Syria, Afghanistan — there are a lot of candidates that are more moderate,” one House Democratic aide told me. “But Yemen is one case where if Sanders can point to it, he can draw distance.”

This matters; as Vox’s Matt Yglesias writes, foreign policy — from military deployments to diplomacy — remains an issue area “where the president faces the fewest institutional constraints.”

That said, while Sanders and Warren have explained their visions for this new era of progressive American foreign policy, Obama proved it doesn’t always play out in practice.

“Right now, progressive Democrats are at the stage of calling into question the conventional answers of the past and coming up with new ones,” Malley said. “What comes next will be harder: translating these theoretical answers into actual policy.”

For example, how would President Sanders fight terrorism without engaging in a forever counterterrorism war? How would he end the war in Afghanistan without betraying the Afghan people? How would he maintain relations with undemocratic countries?

“Well, I’ve not been president yet,” Sanders said, when I asked him how his idea for a values-based foreign policy plays out in practice.

“Talk to Obama. He’ll give you a better answer.”