President Donald Trump vetoed Congress’s bipartisan resolution to withdraw US support from the Saudi-led war in Yemen, cementing American involvement in a deadly humanitarian crisis abroad.

In a historic move, Congress passed a War Powers Resolution in early April, directing the president to remove troops involved in “hostilities” abroad if there has been no formal “declaration of war or specific statutory authorization” from Congress. It was a clear rebuke of Trump’s foreign policy toward Saudi Arabia.

The war in Yemen, directed by Saudi Crown Prince Mohammed bin Salman (or MBS, as he’s commonly referred to in Washington), has killed more than 50,000 people and left more than 20 million Yemenis in need of humanitarian assistance.

The US helps the Saudi-led coalition, which also includes the United Arab Emirates and several other Gulf Arab and African countries, by providing them with intelligence, selling them arms and ammunition, and, until late last year, fueling warplanes.

That means the US is partially culpable for the death and destruction of Saudi’s enemies in the war — the Iranian-backed Houthi rebels — and the thousands of civilians who have been killed. In one particularly egregious incident, a coalition warplane hit a school bus last August with an American-made bomb, killing at least 40 children.

Trump saw the resolution as a personal assault on his authority, adding that the resolution would hurt military efforts abroad.

“This resolution is an unnecessary, dangerous attempt to weaken my constitutional authorities, endangering the lives of American citizens and brave service members, both today and in the future,” Trump said in a memo to the Senate signaling his veto.

The Constitution, of course, gives war declaration powers to Congress, not the president.

Passing the War Powers Resolution through Congress, an effort led by Sens. Bernie Sanders (I-VT) and Mike Lee (R-UT) in the Senate and Rep. Ro Khanna (D-CA) in the House, took immense political capital. It was the result of progressive antiwar activism and a rare bipartisan coalition of progressive and conservative lawmakers to claw back war-approving authority from the president. The final resolution passed in the Senate with the support of seven Republicans, and in the House with the support of 16 Republicans, including some of Trump’s closest conservative allies.

The war powers resolution really caught bipartisan steam last year — closely tied to the shock and outrage over the killing of Saudi journalist, dissident, and American resident Jamal Khashoggi and Trump’s sympathetic response to Saudi interests. Trump has 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.

Trump’s administration and Republican leadership campaigned hard against the War Powers Resolution. That said, lawmakers in support of the push saw a non-zero chance that Trump himself, who campaigned on an antiwar message, could be persuaded.

But that distant hope was ultimately defeated by Trump’s veto Monday night.

“I am disappointed, but not surprised, that Trump has rejected the bi-partisan resolution to end U.S. involvement in the horrific war in Yemen,” Sanders said in a statement.

During a Fox News 2020 presidential town hall Monday, Sanders looked directly into the camera and urged Trump to sign the resolution. Trump spent most of Tuesday disparaging Sanders, and his performance at the town hall, on Twitter. There was a feeling on Capitol Hill that Trump’s veto to the Yemen resolution was a reaction to that.