Yemen’s 21-month war has devastated the country and sparked a humanitarian catastrophe. The UN recorded 4,014 killed and thousands more injured by Saudi-led coalition air strikes between March 2015 and September 2016, carried out with the backing of the US and UK.

The war has displaced 2.2 million Yemenis, while an additional 180,000 have fled the country (refugees are so desperate, some are fleeing to Somalia). Yemen, already one of the poorest countries in the Middle East, was pulled into this crisis after Houthi rebels ousted President Abdrabbuh Mansour Hadi in early 2015. Alarmed by the Houthi rebel advances in Yemen, neighboring Saudi Arabia and its allies in the six-member Gulf Cooperation Council called for the United Nations to bring an end to the “coup” (pdf).

The Saudi-led coalition quickly began an extraordinary air campaign, pounding the rebels in Yemen in a desperate bid to reinstate Hadi’s government. Within three months, the Saudi Arabia-led coalition had been accused of war crimes, including hitting a Médecins Sans Frontières hospital.

That’s a problem for the US and UK, who have been selling weapons to Saudi Arabia. Both countries have signed the Arms Trade Treaty (ATT), which prohibits the selling of weapons where it is known that they would be used in war crimes. The UK, which has ratified the ATT, is bound by its rules, while the US cannot undermine its objective as a signatory.

“In total, Human Rights Watch has documented the use of US weapons in 23 apparently unlawful coalition airstrikes,” says Priyanka Motaparthy, a senior emergencies researcher for Human Rights Watch. “That’s quite a significant number.” Motaparthy also slams the British government for ignoring “overwhelming evidence” that there is a high likelihood that UK-made weapons “could be used in unlawful strikes.”

So, what does this “overwhelming evidence” look like?

Over the course of the war, the UK and US have rebuked Saudi Arabia, but last week the US went one step further and announced it was limiting arms sales to Saudi Arabia amid concerns over Yemen, with a White House spokesman warning Saudi Arabia that US security co-operation was “not a blank check.” (Saudi Arabia would later try to downplay this report).

“This is the first time you have US officials saying ‘because of our concerns about the number of civilian deaths, because of our concern about how the targeting practices. We are halting this sale,'” says Motaparthy, “I think that message—even if only one sale was halted—is an important one.”

But the US is continuing to provide a huge package of military equipment, assistance, and advice to the Saudi Arabia, Motaparthy explains. And despite the evidence of the coalition using cluster bombs, the UK reaffirmed its support for Saudi Arabia, insisting the weapons were used against “legitimate military targets.”

“The US government is the largest arms exporter in the world, so if even it has reservations then you know it’s time to act,” says Andrew Smith, a spokesperson for Campaign Against Arms Trade. “Like the US, the UK has licensed billions of pounds worth of arms to Saudi forces. Like their US counterparts, UK arms companies have fueled and profited from the destruction taking place.”