Samuel Oakford and Ryan Goodman discovered that the U.S. failed to charge the Saudis and Emiratis properly for refueling their planes, so in addition to aiding in their war crimes the U.S. has been subsidizing them as well:
President Donald Trump, who repeatedly complains that the United States is paying too much for the defense of its allies, has praised Saudi Arabia for ostensibly taking on Iran in the Yemen war. It turns out, however, that U.S. taxpayers have been footing the bill for a major part of the Saudi-led campaign, possibly to the tune of tens of millions of dollars.
Given the president’s endless complaints about being ripped off by other countries, it is fitting that he presided over almost two years of letting the U.S. get ripped off by the Saudis and Emiratis at the same time that he has kept the U.S. involved in a disgraceful war. 
U.S. refueling coalition planes stopped as of last month thanks to mounting public and Congressional pressure. Increased Congressional scrutiny of U.S. support for the war is the only reason that we now know these new details. The full costs of U.S. support for the war on Yemen still aren’t known, but this brings us a little closer to a proper reckoning.
Before they ended, the refueling operations were opaque. It was difficult to determine how much fuel the military was providing to coalition planes for its bombing campaign in Yemen. Oakford reported on this in 2017:
But U.S. Central Command, or CENTCOM, now admits that it doesn’t even know how much fuel it offloads for Saudi Arabia and its partners — directly contradicting information about refueling operations that it previously released. Responding to questions from The Intercept, CENTCOM now says that it lumps together refueling data for the coalition with data for U.S. planes in the area, joint U.S.-Emirati missions, and possibly other operations. Even this pooled data has unexplained discrepancies.
In other words: The U.S. military says it doesn’t know how much of its own fuel goes to an indefinite number of operations.
The military then claimed that they weren’t tracking what the refueled planes went on to do. Secretary Mattis also insisted that ending refueling operations would lead to higher civilian casualties, which never made any sense in light of the coalition’s repeated and increasing attacks on civilian targets. Despite Mattis’ assertions, the military didn’t know how their refueling support was being used, and they weren’t inclined to find out. Now it turns out that there has effectively been no formal agreement with the Saudis the entire time that the refueling was going on:
After the first year of the Yemen war, the U.S. drafted a provisional ACSA with the Saudis, but the Pentagon says that Congress was never notified because the Kingdom, even today, hasn’t “fulfilled all of its internal procedures necessary for an Agreement to enter into force.” In short, throughout the entire duration of U.S. refueling, the Pentagon admits there was never an official servicing agreement in force with Saudi Arabia.
The military failed to meets its obligations under two different administrations, and Congress failed until very recently to do the necessary oversight that they should have been doing all along. The U.S. has not only been enabling Saudi coalition war crimes in an unauthorized war for more than three and a half years, but American taxpayers have also been paying far more for the war than we knew.
https://www.theamericanconservative.com/larison/the-great-saudi-rip-off/