Ending American assistance to the Saudi-led coalition fighting in Yemen would curtail Riyadh's war efforts and hasten the end of what the United Nations describes as the world's worst humanitarian crisis, experts say.

A push by US lawmakers to end support for the war once appeared largely symbolic, with only a few Democrats in the Republican-controlled Congress putting forward a proposal, but now legislators may be set to pass a measure that would halt US assistance to Saudi-led forces in Yemen.

That would have a critical impact, said Robert Jordan, former US ambassador to Saudi Arabia in the early 2000s, who described US support as crucial to Riyadh's military capabilities.

"If we suspend providing spare parts for their F-15s, their air force would be grounded in two weeks," Jordan told Middle East Eye last week. "So I think there is every prospect that, if that occurs, they will find it more appealing to go to the peace table and negotiate than they currently do."

The proposed US legislation cleared the House of Representatives last month, and the Senate, which approved a similar motion late last year, is expected to vote on it again in the near future.

The bill invokes the War Powers Resolution of 1973, which prohibits the involvement in a foreign conflict without congressional authorisation. President Donald Trump has vowed to veto the legislation, which would require a two-thirds majority in the Senate to override.

'Extremely important'

Khalil Jahshan, executive director of the Arab Center Washington DC, said both Washington and Riyadh would like to downplay the impact of American involvement in Yemen, but the US role in the war remains "extremely important" logistically and politically.

Beyond helping with military assistance, Washington provides "psychological and strategic cover" to Saudi war efforts, he said.

"If it weren't for American support, if that were to be withdrawn in the future ... I think Saudi Arabia would feel compelled to end that war faster than they would like," Jahshan said.

While Trump is often criticised for his cozy relationship with Saudi Arabia's rulers, the conflict in Yemen started under his predecessor, Barack Obama.

Saudi Arabia and the United Arab Emirates started a massive bombing campaign in Yemen in 2015 to restore the government of President Abd Rabbuh Mansour Hadi after Houthi rebels captured the capital, Sanaa.

If the US turns off the tap to the Saudis and Emirates ... the pressure to end the war would be very hard to resist -Nabeel Khoury, former US diplomat

While the congressional bill does not prohibit military sales, US lawmakers are poised to scrutinise future Saudi purchases of US weapons, fuelled by anger following the assassination last October of Washington Post columnist Jamal Khashoggi by a Saudi hit-squad.

US help to the Saudi-led coalition in Yemen has included mid-air refuelling of fighter jets, aid with a naval blockade and help coordinating military operations, among other things.

Nabeel Khoury, a former US diplomat who served in Yemen from 2004 to 2007, said that assistance has been vital to Saudi Arabia's military plans.

"That these two regimes depend heavily on US support is undeniable," Khoury said of Saudi Arabia and the United Arab Emirates. "Definitely at the beginning, they could not have started this war without US and UK support."

However, over the course of the war, Riyadh and Abu Dhabi have developed their own military capabilities and hired private experts and mercenaries to help them, he said.

Indeed, when Washington halted mid-air refuelling last year, the Saudis acquired the capacity to do it on their own.

Khoury, now a non-resident senior fellow at the Atlantic Council, said ending Washington's assistance, including arms sales, would force Saudi Arabia to end the conflict.

He cited Trump's comments from last year, when the president said Saudi Arabia "might not be there for two weeks" without US support. "It is in the Saudis' ability to stop the war if they genuinely wanted to," Khoury told MEE.

Humanitarian chiefs concerned over Saudi and UAE aid funding for Yemen Read More »

"If the US turns off the tap to the Saudis and Emirates, and the Saudis and Emirates turn off the tap for the people in Yemen that they support, including the hired mercenaries from all over the world, the pressure to end the war would be very hard to resist."

Still, the former diplomat warned that the situation in Yemen is complicated, with conflicting agendas and several parties involved, including Houthi rebels, southern separatists, government loyalists, followers of former president the late Ali Abdullah Saleh and hardline militants including al-Qaeda.

"It's not like you could say, 'Stop everything' today and tomorrow everything stops," Khoury said. "But essentially, without American and British support the air coalition would find it very hard to continue with this war."

What is US policy in Yemen?

In late 2018, both US Secretary of State Mike Pompeo and then-secretary of defense James Mattis called for an end to the conflict in Yemen.

Despite this, Washington has done little to pressure its Saudi and Emirati allies to reach a lasting peace.

US policy on Iran: What is the endgame? Read More »

Khoury said that when he was in the US foreign service in Yemen, the sole focus of US policy was counter-terrorism, a strategy that he said he tried to broaden by pushing for economic aid to the impoverished country.

Under the Trump administration, the US views Yemen as part of the country's confrontation with Iran, Khoury said. "This administration took on the concern for counter-terrorism, and the military is continuing to do that, but they added the Iran factor times 10."

Riyadh accuses the Houthis of being Iranian proxies, but Tehran denies providing material support for the Yemeni rebels.

"They look at Yemen, and they see Iran. They don't see Yemen at all," Khoury said of the Trump administration. "And that's a big, big mistake and a big part of the problem for both Saudi Arabia and the US."

If we suspend providing spare parts for their F-15s, their air force would be grounded in two weeks - Robert Jordan, former US ambassador to Saudi Arabia

He said that even when Pompeo and Mattis called for ending the conflict, American hawks fixated on Iran, including National Security Advisor John Bolton, continued to push for assisting the Saudis in a perceived struggle against Tehran.

"If [Bolton] hasn't had enough coffee in the morning - or has had too much coffee during the day - he sees red, and red is Iran for him," Khoury said.

Jahshan, of the Arab Center, also said that Trump administration officials are not on the same page when it comes to Yemen.

"The White House is not convinced," he said. "Their preference is to continue to support the Saudis for the same reasons that justified their position on Khashoggi and all kinds of other issues.

"Basically, they do not want to cross the Saudis ... and impact their ability to buy weapons and spend money in the United States."