President-elect Joe Biden could dramatically improve the worst humanitarian crisis in the world within hours of taking office in January with one simple step: ending American support for the Saudi-led military campaign in Yemen. Without the U.S.s backing, the Saudis and their partners would do far less damage in the Middle Eastern country, where prolonged conflict has killed more than 17,500 civilians and pushed 10 million people to the brink of starvation.

On Monday, 80 advocacy groups urged Biden to take that step as soon as he can, securing what they call a monumental first achievement for your administration. The groups shared their message to the incoming president exclusively with HuffPost after they sent it to him.

As the groups noted, this would be a major step by Biden, who backed the war while he served as vice president for President Barack Obama, but has since vowed to bring it to an end. Many senior Obama administration officials who will play key roles in his team, like incoming national security adviser Jake Sullivan, have also called for the U.S. to withdraw.

The groups want Biden to follow through and fast, which would demonstrate that he prioritizes a less belligerent foreign policy.

The Biden-Harris administration has an historic opportunity to end U.S. complicity in this war the moment you come into office, the letter reads.

With countless new deaths from war and starvation every single day, the people of Yemen cant afford to wait, the groups add.

The letters signatories range from anti-war groups such as CODEPINK and Peace Action to progressive organizations working on various issues like the Center for International Policy and Daily Kos; a number of faith organizations also endorsed the message.

They want Biden to quickly carry out a set of interconnected policy changes. They argued Biden should cut off logistical and intelligence assistance to the Saudis by signing bipartisan legislation Congress has repeatedly supported; end arms sales to the Saudis and nations working with them; pressure the Saudis to end their bombing and their blockade of the country; and restore American humanitarian aid for Yemenis, which Trump slashed as the coronavirus pandemic set in .

Ending U.S participation would signal to millions of Yemenis living in Yemen and thousands of Yemeni-Americans who worry about their families in Yemen that weapon sales and geopolitical chess moves are not more important than their lives and the lives of their loved ones, the message reads.

Biden repeatedly condemned Trumps warm relationship with Saudi Arabia on the campaign trail, saying he would instead treat the kingdom as a pariah for its human rights abuses in Yemen and elsewhere, like the assassination of journalist Jamal Khashoggi in Turkey.

Withdrawing from the war would count as an election promise fulfilled and represent a sharp turn from Trump, who continued the support despite bipartisan criticism from Congress, even rushing through dubious emergency declarations to overrule lawmakers who were blocking arms transfers.

If he withdrew the U.S. from the conflict, Biden would finally accomplish a goal that Democrats and their allies have worked toward for years . He would also show that he has learned from the Obama era and is willing to recognize that they got some things wrong. And he would simultaneously thrill more liberal Democrats who are leery of his centrist record and secure a rare moment of unity with congressional Republicans; some key GOP figures broke with Trump and Senate Majority Leader Mitch McConnell (R-Ky.) to join the fight against the Yemen war.

For Biden and for the U.S., which has seen its global standing plummet under Trump, a change on Yemen could be the first step to showing Washington will approach the world differently. The U.S. needs to prioritize human rights in our foreign policy, said Yasmine Taeb of the Center for International Policy.

Its almost certain that Biden will make good on the pledge to withdraw from Yemen. But the way he does so will be telling: for Biden to act as the activists asked him to on Monday, by signing legislation acknowledging limits on his presidential powers on foreign policy and ending most arms sales to the Saudis and their friends like the United Arab Emirates, would be a leftward shift.

Alternatively, he could cut the support for the war while restricting some but not all military assistance to those partner nations, as more conservative advisers may suggest. Some in Bidens orbit feel a public fight with those countries could backfire if they take more aggressive actions themselves, or if Iran, a shared adversary for them and the U.S., tries to benefit from a split.

Meanwhile, Trump remains commander in chief for more than a month and could complicate Bidens plans. He is advancing a gigantic arms deal for the U.A.E. that will embolden that country, a close Saudi ally. And he is considering labeling the Houthis, the Iran-linked group that the Saudis are fighting in Yemen, a terror organization, which would make it significantly harder to hold peace talks with them and send aid to areas they control, where more than 70% of Yemenis live.

Pulling the U.S. out should be among Bidens top priorities for his first days in office, Ariel Gold, CODEPINKs national co-director, told HuffPost.

A Biden spokesman declined to comment on his plans, citing the principle that the U.S. has only one president at a time.