The U.S. Capitol in Washington. (Bloomberg News)

Something startling happened in the Senate this week. Taking on both President Trump and the government of Saudi Arabia, the chamber voted 63 to 37 to advance a resolution that would end U.S. military support for the Saudi-led war effort in Yemen. If Congress approves the resolution — which is opposed by Trump and still faces major legislative hurdles — it would mark the first time that lawmakers have formally terminated a U.S. military engagement since ending the U.S. war in Vietnam. And it would signal quite strongly that Congress will no longer tolerate business as usual with Saudi Arabia.

Here’s what you need to know.

How startling a change was this vote?

The Senate vote was a remarkable change. Just eight months ago, only 44 senators voted to advance the same resolution to pull the United States out of the war being waged by Saudi Arabia and the United Arab Emirates against the Houthis in Yemen. That resolution was introduced by Sens. Bernie Sanders (I-Vt.), Mike Lee (R-Utah) and Chris Murphy (D-Conn.). And just a couple of years ago, few on Capitol Hill paid any attention to U.S. involvement in Yemen.

What’s more, Congress has studiously avoided taking any action to update or repeal the authorization for use of military force (AUMF) that passed soon after the Sept. 11, 2001, terrorist attacks — and which presidents ever since have relied on to intervene militarily in Afghanistan, Iraq, Yemen and more. Some lawmakers have tried to repeal it; others have tried to replace it. But most members of Congress have preferred to stay away from military decisions entirely, letting the president take all the credit or blame.

So why are senators looking at ending U.S. involvement in the Yemen war now?

Senators are angry about two major developments.

First, the Saudi-led coalition’s most recent offensive killed hundreds of Yemeni civilians. At the same time, the Yemeni humanitarian crisis — the worst in the world — has gone from very bad to even worse, with millions of people displaced and facing starvation and disease. Human rights groups, aid organizations and journalists have made it difficult to ignore the scale of the ongoing disaster.

Second, senators are disturbed by the Saudi government’s murder of Washington Post contributing columnist Jamal Khashoggi, which the CIA has concluded was ordered by Crown Prince Mohammed bin Salman. In part because Trump has refused to accept the CIA’s conclusion, many members of Congress want to send Saudi leaders a strong message that the United States will not tolerate its reckless behavior and violations of basic international norms. As a result, increasing numbers of both Republican and Democratic lawmakers support pulling out of the military effort in Yemen or otherwise taking steps to distance the United States from Saudi Arabia, such as restricting arms sales to the country or imposing sanctions on the senior Saudi officials responsible for Khashoggi’s murder.

That brings us to the vote to open debate on the Sanders-Lee-Murphy resolution. Fourteen Republicans joined all 49 Democrats and independents in supporting it. What’s interesting is that the resolution’s chief sponsors come not from both parties’ moderate wings but from the progressive left and libertarian right. That’s similar to the strange bedfellows coalition behind recent efforts to advance criminal justice changes. Bipartisanship doesn’t always emerge from the political center.

How unprecedented would this congressional step be?

The Constitution and the 1973 War Powers Resolution require Congress to authorize any U.S. decisions to go to war. But for years now, presidents have deployed the military without congressional approval. Examples include President Ronald Reagan’s interventions in Grenada and Lebanon; President Bill Clinton’s interventions in Bosnia and Kosovo; President Barack Obama’s intervention in Libya; and military support by Obama and Trump for the Saudi-led war in Yemen.

Congress wasn’t necessarily enthusiastic about many of these military actions. But Capitol Hill has not passed legislation formally terminating U.S. involvement in a conflict since it forced an end to U.S. military activity in Vietnam, Cambodia and Laos in 1973. In most other cases, Congress has been too divided to take decisive action. Many lawmakers defer to the president and military leaders during wartime; that tendency makes it still more difficult to win over majorities to go on record in favor of ending military efforts.

That said, Congress has used tools other than legislation to influence U.S. military engagements, including using its power of the purse to curtail presidential ambitions abroad. And presidents tend to be very attentive to the views of Washington elites, often adjusting military decisions because of the reactions they expect from Capitol Hill. Congressional statements and hearings sometimes shape public attitudes, pressuring presidents to shift course.

What might happen next?

The Sanders-Lee-Murphy resolution is unlikely to pass the Senate and House in its current form. Secretary of Defense Jim Mattis and other senior administration officials argue that the resolution would damage relations with an important security partner. Most Republican lawmakers remain unwilling to support the resolution. Even if the Senate approves it, House Republican leaders have already moved to block action on a related measure.

That means lawmakers would need to start from scratch in the new Congress — where a Democratic-led House is more likely to bring such a resolution to a vote.

Whether that resolution passes, watch for other congressional action rebuking Saudi Arabia, such as a more limited reduction in U.S. support for the Yemen war effort, arms sales restrictions or new sanctions. Activist lawmakers may push to include such provisions in the appropriations legislation that Congress must soon pass to avoid a government shutdown.

In the meantime, the mere fact of this week’s vote sends two powerful signals. First, the Senate is declaring that the United States considers human rights and the rule of law to be extremely important. And second, U.S. lawmakers are becoming more willing to assert their constitutional prerogatives as a coequal branch of government.

Jordan Tama is associate professor at American University’s School of International Service and is the co-editor of the sixth edition of “Rivals for Power: Presidential-Congressional Relations” (Rowman and Littlefield, 2018). Follow him on Twitter @ProfJordanTama.