Fuelled by outrage over Saudi Arabia's murder of journalist and U.S. resident Jamal Khashoggi, Congress on Thursday took a historic vote to rebuke the kingdom by ending U.S. involvement in the Saudi-led war in Yemen, the world's worst humanitarian catastrophe.

The 56-41 vote in the Senate advanced a measure to cut support for the Saudi-led coalition battling an insurgency in Yemen. A handful of Republicans took the rare step of publicly defying President Donald Trump's administration, siding with Democrats to punish Riyadh.

The bill marks the first time a measure linked with the 1973 War Powers Resolution was debated, then voted on in either chamber of Congress. Its passage is a black eye for the president.

"The fact Republicans are willing to move forward to endorse the measure is pretty damaging to the Trump administration," said Scott Anderson, a national-security and Mideast expert with the centrist Brookings Institution in Washington. "Republicans don't relish taking on this sort of issue."

The rebuke is a damning condemnation of Saudi Arabia from the American legislative branch, though it's largely symbolic because it won't likely hit the House floor in this session.

Here's what you need to know about the resolution.

What's going on in Yemen?

An estimated 17,000 civilians have been killed in the civil war, which has roots in the 2011 Arab Spring.

The conflict escalated after the Houthi rebels, a Shia group, waged a coup that overthrew the government in 2014.

Fearing a growing Shia Muslim influence at its doorstep, the Sunni kingdom of Saudi Arabia, Yemen's neighbour, launched airstrikes backed by U.S. logistical aid.

Millions have been displaced by war in northwestern areas of Yemen, like this family sitting in their makeshift hut on a street in the Red Sea port city of Hodeida, Yemen, Dec. 24, 2017. (Abduljabbar Zeyad/Reuters)

Saudi's chief rival in the region is Iran, a Shia-majority powerhouse that is backing the Houthis with ballistics supplies.

Some Mideast experts have characterized the bloody stalemate with the Houthis as a proxy war with Iran.

Exacerbating the humanitarian crisis has been a Saudi-imposed economic blockade that the United Nations says has brought about the deadliest famine in a century. Some eight million people are on the verge of starvation and 20 million face food insecurity, according to the World Food Programme.

How is the U.S. involved?

American-supplied bombs have killed Yemeni civilians, including children.

U.S. intervention in Yemen began under Barack Obama's presidency. It was officially justified at the time "to defend against Houthi violence." But it was reportedly an act of appeasement for Riyadh agreeing to hold its nose over U.S. support for the Iran nuclear deal.

The U.S. is mainly interested in defeating al-Qaeda and any terrorism threats coming out of Yemen, as well as restricting the possible expansion of Iranian influence, said Katherine Zimmerman, research manager for the Critical Threats Project at the American Enterprise Institute, a conservative-leaning think-tank. A Houthi maritime threat and merchant shipping security is another U.S. security interest.

The Trump administration has expressed concerns about wanting to contain Iran's influence in the Middle East, and has viewed its alliance with Saudi Arabia as key to constraining Iran, which the U.S. considers a state sponsor of terrorism.

Trump has resisted calls to admonish Saudi Arabia's Crown Prince Mohammed bin Salman, seen here in the Oval Office in March, for his alleged role in the killing of Jamal Khashoggi. (Mandel Ngan/AFP/Getty Images)

In 2015, the White House authorized "logistical and intelligence support" for the Sunni-dominated Gulf Co-operation Council's efforts. Aside from intel-sharing, American logistical support includes advice and training and, until last month, involved midair refuelling of Saudi-led aircraft. The U.S. has also supplied precision-guided munitions and sold billions of dollars' worth of weapons to Saudi Arabia.

Military experts doubt the Saudis could conduct the level of operations they have so far without this American support.

What's the Senate vote about?

The bill's wording directs the Trump administration to pull U.S. forces from "hostilities" in Yemen, pursuant to the War Powers Resolution. Its co-sponsors were an unlikely trio: Independent self-described democratic socialist Bernie Sanders, Tea Party conservative Mike Lee and Democrat Chris Murphy.

Thursday's vote is the first time Congress has invoked a War Powers resolution — a Vietnam-era law that restricts the president's ability to send troops into battle — and seen it advance in the Senate.

It followed a historic 60-39 vote on Wednesday to begin debate on the measure, a stage that itself was "pretty exceptional for getting this far," Anderson said.

What's the Trump administration saying?

Trump has refused to condemn Saudi Crown Prince Mohammed bin Salman for Khashoggi's killing, despite the CIA assessment that the Saudi government ordered the murder.

U.S. Secretary of State Mike Pompeo has backed the president, maintaining "there is no direct reporting connecting [bin Salman] to the order to murder Jamal Khashoggi."

Why the sudden momentum?

A similar procedural vote actually failed back in March. What's changed is a new surge of support from Republican senators enraged by Khashoggi's slaying and Trump's refusal to accept his own intelligence community's conclusion.

"Had the Khashoggi incident not occurred ... I doubt that key Republicans would have broken with the president to bring this to a vote," said Derek Chollet, who served for six years in the Obama administration at the State Department and the Pentagon.

Will this actually end the U.S. role in Yemen?

Odds look slim. One sticking issue is whether the U.S. is in direct "hostilities" in Yemen, as the resolution states.

"There's a grey area. The U.S. is not involved in direct military operations but providing critical support to the Saudis in terms of their own military options," Chollet said. But he noted "hostilities" isn't defined in the War Powers legislation.

That said, selling munitions or refuelling Saudi planes "is certainly aiding and abetting," he said.

Jamal Khashoggi, a Washington Post contributor, was killed in October after a visit to the Saudi consulate in Istanbul to obtain paperwork before marrying his Turkish fiancée. (Yasin Akgul/AFP/Getty Images)

While the Pentagon has said the current U.S. participation doesn't meet the standard of direct engagement, retired Lt.-Col. Dan Davis calls that "absurd."

"That's like saying, 'I drove the getaway car to the bank robbery, but I didn't rob the bank.' Well, yes, you're still complicit," said Davis, a military expert with the libertarian Defense Priorities Foundation.

Are there other hurdles?

Thursday's vote is largely a symbolic shot across the bow to Trump because the House will not vote on it before the end of this session.

House Republicans signalled they were not interested in passing any such resolution when they inserted an obscure provision into the House Farm Bill on Wednesday that was narrowly adopted.

A boy stands on the rubble of a house destroyed during the conflict in the northwestern city of Saada, Yemen. (Naif Rahma/Reuters)

Any measure that isn't finalized by the president before Congress closes this year will become null and void, and will have to be restarted from scratch in the next session.

The Yemen resolution has a better chance of passing when the new Democratic-controlled House takes over in January.

"The Democratic leadership will be much more willing to entertain this in the new year," Anderson said, as top party leaders, including the presumptive next House speaker Nancy Pelosi, have endorsed it.

But there's a wrinkle: The new Senate configuration has strengthened the Republican presence to 53 seats from the current 51, and senators more open to entertaining the idea — notably Jeff Flake and Bob Corker — will be departing.

One way or another, this week will be seen as an inflection point in U.S.-Saudi relations.

"It became so crystal-clear that Republicans are willing to take a stand publicly and push for this, and that's what you need before you can get any law in place," Anderson said. "There's a real legacy here. People will look at this act as the tipping point toward the end of U.S. involvement in this conflict."