WASHINGTON — As global attention focused on Russia's invasion of Ukraine earlier this year, the Saudi-led coalition carried out more than 150 airstrikes on civilian targets in Yemen, including homes, hospitals and communication towers, according to the Yemen Data Project. It was the latest uptick in bombing during a grinding, and often overlooked, civil war that has upended the lives of Yemeni civilians for the better part of a decade and spawned one of the world's most severe humanitarian crises.

Hundreds of thousands have died from the fighting or its indirect consequences, such as hunger, the United Nations says. The devastating air campaign alone — carried out by a Saudi-led coalition — has killed nearly 15,000 people, according to conservative estimates by the Armed Conflict Location and Event Data Project (ACLED), which monitors war zones around the world.

While Russia's bombings of a maternity hospital and other civilian targets in Ukraine have drawn widespread public indignation as war crimes, thousands of similar strikes have taken place against Yemeni civilians. The indiscriminate bombings have become a hallmark of the Yemen war, drawing international scrutiny of the countries participating in the air campaign, and those arming them, including the United States. U.S. support for the Saudi war effort, which has been criticized by human rights groups and some in Congress, began during the Obama administration and has continued in fits and starts for seven years.

New analysis by The Washington Post and Security Force Monitor at Columbia Law School's Human Rights Institute (SFM) provides the most complete picture yet of the depth and breadth of U.S. support for the Saudi-led air campaign, revealing that a substantial portion of the air raids were carried out by jets developed, maintained and sold by U.S. companies, and by pilots who were trained by the U.S. military.

The Biden administration in 2021 announced an end to U.S. military support for "offensive operations" carried out by the Saudi-led coalition against Yemen's Houthi rebels and suspended some munition sales. But maintenance contracts fulfilled by both the U.S. military and U.S. companies to coalition squadrons carrying out offensive missions have continued, The Post's analysis shows.

The Post and SFM reviewed more than 3,000 publicly available images, news releases, media reports and videos identifying for the first time 19 fighter jet squadrons that took part in the Saudi-led air campaign in Yemen. More than half of the squadrons that participated in the air war came from Saudi Arabia and the United Arab Emirates — the two countries that carried out the majority of the air raids and receive substantial U.S. assistance.

An analysis of public contract announcements shows that the United States provided arms, training or maintenance support to the majority of the fighter jet squadrons in the campaign. The Post found that as many as 94 U.S. contracts were awarded to individual Saudi and UAE squadrons since the war began.

Despite Pentagon statements that it is difficult to pinpoint which units in foreign militaries receive U.S. assistance, The Post-SFM analysis identified specific airstrike squadrons that received U.S. support, proving the universe of squadrons carrying out airstrikes is a narrow and knowable one.

"For most coalition countries, there is no way for [America] to support their planes without supporting squadrons that may be linked to airstrikes that human rights groups say are apparent war crimes," said Tony Wilson, the director of Security Force Monitor.

The analysis revealed that 39 squadrons from Saudi-led coalition member states flew aircraft with airstrike capabilities. The majority of these units flew fighter jets that were developed and sold by American companies.

The Post and the Security Force Monitor used visual evidence from state media, news reports and government releases to identify 19 fighter squadrons that definitely took part in the air campaign in Yemen since 2015.

A review of more than 900 publicly available sales announcements revealed that the four squadrons from Saudi Arabia that fly F-15S/SA planes benefited — and the remaining 15 squadrons probably benefited — from U.S. weapons and equipment contracts signed after the start of the war.

At least one squadron each from Bahrain, Kuwait and the UAE benefited from U.S. contracts because of support going to a type of plane, but lack of specificity in information published by the Department of Defense makes it impossible to know the exact squadron.

The Post additionally reviewed more than 1,500 videos, photos and public statements by the Department of Defense and coalition members since the war began and found the United States participated in joint exercises with at least 80 percent of squadrons that flew airstrike missions in Yemen. At least four times, these exercises took place on U.S. soil.

In some instances, The Post and SFM could only determine that certain squadrons were likely to have benefited from U.S. contracts. Sales announcements never name specific squadrons that will benefit, only a type of plane or piece of equipment being sold. Thus for certain squadrons, The Post and SFM could only determine probability because every coalition country has at least two airstrike squadrons flying the same type of plane.

The Saudi-led coalition and every member state except Qatar did not respond to The Post's request for comment on the report's findings. A Qatar official familiar with the country's role in Yemen told The Post that Qatar left the coalition in June 2017, but they did not answer questions about the country's involvement in the air raids over Yemen.

When presented with the findings, the Defense and State departments pointed to the steps the Biden administration had taken to end the war in Yemen, the U.S. decision to end aerial refueling for coalition aircrafts in 2018 and the ongoing trainings to reduce civilian casualties.

"America's alliances and partnerships are our greatest asset, and so we are committed to standing shoulder-to-shoulder with our key partners in the Middle East," said Army Maj. Rob Lodewick, a Pentagon spokesman. But, he acknowledged that "considerable work remains to be done" with the Royal Saudi Armed Forces's targeting procedures and investigative capacity.

"Both [Saudi Arabia and the United Arab Emirates] face significant threat to their territories," State Department spokesman Ned Price told The Post, noting that the Houthis had launched hundreds of cross-border attacks on Saudi Arabia in just the last year. "We are committed to continuing to strengthen those countries' defenses," Price said.

The contracts reviewed for the analysis are only a small fraction of total U.S. arms sales to coalition countries. The specifics of certain sales are never released to the public. One such case is a direct commercial sale where American companies sell directly to governments, as opposed to foreign military sales where the U.S. government is the seller. Others — including arms deals that are valued at less than $14 million — do not require congressional review and so are not generally publicly announced.

Evidence of previous human rights violations is rarely enough to halt sales, former State Department officials told The Post, in part because the sheer volume of the contracts overwhelm human rights concerns in the vetting process.

"The staffing constraints meant that, over the course of a year, the State Department was expected to complete an analysis on human rights risks associated with a weapons export license every 5 minutes," House Democrats wrote in an April 27 letter to a House Appropriations subcommittee.

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Seven years and three American administrations into the war, each of the airstrike-capable squadrons from Saudi Arabia and the UAE received or is likely to have received U.S. weapons and support. U.S. forces conducted joint exercises with almost every squadron from Saudi Arabia and the three F-16E/F squadrons from the United Arab Emirates confirmed to have flown missions in Yemen.

In videos broadcast by the Emirates News Agency, two UAE squadrons — the 1st and 2nd Shaheen — were frequently shown taking off loaded with air-to-surface missiles for airstrike missions against the Houthis in Yemen. The UAE has also taken part in a separate campaign with the United States against al-Qaida in the country. News reports and visual evidence show the same F-16E/F squadrons and an additional F-16E/F squadron — the 3rd Shaheen — participating in joint exercises with U.S. forces at Red Flag exercises hosted at Nellis Air Force Base in Nevada in 2016 and 2019 as well as in the UAE as recently as last year. The U.S. Air Force holds these Red Flag exercises with allies simulating aerial combat several times a year.

Broadcasters reporting from Saudi air bases claimed to show the F-15SA, an American fighter jet sold to the Saudis in 2010 as part of a $29 billion deal, taking off to conduct airstrikes in Yemen as early as 2018. The F-15S and F-15SA fighter jets — flown by Saudi's 6th, 29th, 55th and 92nd squadrons — were regularly promoted by Saudi state media as key to the coalition's air campaign.

The last F-15SA was delivered to Saudi Arabia in 2020, and dozens of contracts supporting the new fleet and the upgrade of the other F-15s were awarded after 2015. A review of annual State Department reports by the Security Assistance Monitor show the Defense and State departments planned sales of approximately $2 million in F-15 trainings for Saudi aviators, including fighter jet trainings, through foreign military sales between fiscal years 2015 to 2020. The reports do not include any trainings that may have been purchased through direct commercial sales.

An analysis of news releases, videos and photos reveals for the first time at least three of these four Saudi squadrons not only received new equipment but participated in at least 13 trainings and joint exercises — including at least one on U.S. soil. A new training unit of F-15 SA fighter jets participated in a Red Flag exercise at Nellis Air Force Base in Nebraska with U.S. pilots as recently as March 2022.

Since 2015, human rights groups investigating the airstrikes have identified more than 300 that violated or appeared to violate international law, according to The Post and SFM's survey of publicly available reports and documents. Although individual squadrons have never publicly been implicated in specific airstrikes, which are always described as being carried out by the coalition, the then-head of U.S. Central Command, Gen. Joseph Votel, confirmed in 2019 testimony that the United States had access to a detailed database of the coalition's airstrikes in Yemen.

"We do have a database that does have that information and we have the ability to see that," he said in response to a question from Sen. Elizabeth Warren, D-Mass., asking if U.S. military personnel based at the Saudi-coalition headquarters readily had access to "a database that detailed every airstrike: warplane, target, munitions used and a brief description of the attack."

The database's existence suggests some American officials had more knowledge of which weapons were used and which squadrons participated in airstrikes leading to civilian harm than the public and members of Congress had been told they had. The U.S. Air Force declined a Freedom of Information Act request by The Post to access the database, claiming it did not have the records.

The United States is prohibited from providing security assistance to units of foreign security forces credibly implicated in the commission of a gross violation of human rights, according to two statutes known as "Leahy Laws" after their main sponsor, Sen. Patrick J. Leahy, D-Vt. However, since the Clinton era, subsequent administrations have interpreted that the vetting of units under these laws only occurs when the security assistance — be it training, equipment or other assistance — is financed by the State Department or Defense Department, said Sarah Harrison, a senior analyst with the International Crisis Group and former associate general counsel at the Defense Department.

Wealthy countries, like Saudi Arabia and the UAE, are not subject to such vetting because they typically pay for all assistance through foreign military sales or direct commercial sales. Leahy "has long insisted that as a matter of policy, it makes no sense to have one such standard for weapons that we given to a foreign security force, and another for weapons that we sell to that same security force," said Tim Rieser, the senior foreign policy aid to the senator.

The Defense Department did not respond to requests from The Post to clarify if and how these units might have been vetted or if Leahy's provisions did not apply ahead of joint exercises or additional weapons deliveries.

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As early as March 2015, U.S. officials worried that coalition airstrikes may have violated the rules of war. Internal State Department documents, written between mid-May 2015 and February 2016 and released as part of a Freedom of Information Act request by Reuters, revealed concern at the State Department about the Saudi-led coalition's airstrikes and the legal implications for U.S. officials.

Aiding and abetting war crimes under international law has been applied differently in courts, including domestic ones. Under one standard, individuals or a state may be found guilty of aiding and abetting if they continued to provide assistance to a problematic actor with knowledge that their support would contribute to future crimes and despite assurances.

"As long as the international humanitarian law violations by the Saudis and U.S. sales to support those operations are both ongoing, there are serious concerns about U.S. complicity in the Saudi war crimes that result," said Oona Hathaway, a professor of law and political science at Yale Law School.

The United States implemented multiple measures aimed at curbing civilian harm beginning in 2016, including sending advisers, adding "civilian casualty avoidance, the law of armed conflict, human rights command and control" training for the Royal Saudi Air Force and by 2019, four years into the war, adopting a policy requiring that precision-guided missiles be sold with appropriate targeting infrastructure.

Human rights observers in Yemen said they did not see any meaningful change in the air campaign as a result of these measures. Airstrikes are still responsible for the vast majority of civilian deaths.

A 2020 review of an emergency transfer of arms to the Saudi-led coalition by the Office of Inspector General at the State Department found in the case of that specific transfer of precision-guided munitions "that the Department did not fully assess risks and implement mitigation measures to reduce civilian casualties and legal concerns."

"The U.S. defense [against aiding and abetting] may be that they are trying to mitigate by working with the most problematic actors," said Hathaway. "But if they attempt mitigation and violations continue, and they still continue support, then that undermines the defense [against liability]."

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Since taking office, the Biden administration has repeatedly made clear ending the war in Yemen is a priority and banned "offensive support" for the coalition. But it has approved sales of "defensive weapons," including a $650 million sale of air-to-air missiles to Saudi Arabia and a $65 million sale to bolster the UAE's missile defense system.

The ongoing maintenance contracts have not been impacted by Biden's policy shift and have drawn sharp criticism from some members of Congress. House Democrats introduced legislation in February to ban U.S. maintenance of planes carrying out airstrikes in Yemen. On Wednesday, a bipartisan group of representatives proposed a war powers resolution to further curtail American involvement in the war.

"If we don't sell the particular ammunition, they can still fly," Rep. Tom Malinowski, D-N.J., who served as assistant secretary of state for human rights during the Obama administration, told The Post. "They have got a lot of munitions stockpiled. They might be able to find replacements today, but there's no replacement for the maintenance contract and no ability to fly without it."

Yemen is in its longest period of no airstrikes under a cease-fire that began during the Islamic holy month of Ramadan and was renewed Thursday. Biden welcomed the continued truce, noting the United States will remain engaged with the diplomatic process over the coming weeks and months.

The months preceding the truce saw the longest sustained period of airstrikes since 2018, according to Iona Craig, the director of the Yemen Data Project, a nonprofit organization that tracks air raids. Strike data showed the escalation began in October 2021, the same month that the United Nations Human Rights Council voted to end its independent investigatory group on Yemen.

Transparency into the world of arms sales — particularly as it pertains to U.S. allies in the Saudi-led coalition — has long been muddied by complex laws, an alphabet soup of government agencies and deep U.S. interests abroad.

Still, "to have the U.S., over successive administrations, sell billions of dollars worth of weapons to governments that have carried out, over years, airstrikes on hospitals, markets, food production facilities and prisons: [Those] attacks have killed thousands of civilians," said Priyanka Motaparthy, director of the Counterterrorism, Armed Conflict and Human Rights Project at Columbia University Law School's Human Rights Institute. "It does not serve them well in the court of public opinion, or in the annals of history."

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The Washington Post's Missy Ryan, Kareem Fahim and Alex Horton contributed to this report.

Database of squadrons compiled by Security Force Monitor at Columbia University. Database of U.S. assistance contracts and joint training exercises compiled and analyzed by The Washington Post and the Security Force Monitor.

Airstrike counts and fatality totals reported by ACLED and Yemen Data Project.

Database of Foreign Military Training and Defense Department training compiled by Security Assistance Monitor.

Discovery and analysis of information in Arabic on the coalition air campaign by Security Force Monitor, Mwatana for Human Rights and The Washington Post.

Data on squadron and plane tail numbers provided by Scramble.