The revelation—detailed in a Defense Department letter obtained by The Atlantic—is likely to raise further ire among senators who have grown ever-more critical of Saudi conduct in the war, which has resulted in a growing number of civilian casualties, and U.S. support for it.

Since the start of the Saudi-led intervention, in March 2015, and up until last month, the United States provided mid-air refueling for Saudi-led coalition aircraft that then flew missions related to the Yemen campaign. Getting heavy U.S. tankers into the air and carrying out this job is enormously expensive. The recipient country is required by law to pay the costs, but that isn’t what happened here. In a mea culpa of sorts, the Pentagon’s November 27 letter states that while the Defense Department “believed” Saudi Arabia and the United Arab Emirates “had been charged for the fuel and refueling services, they in fact had not been charged adequately.” How inadequately, the Pentagon will not yet say; it is “currently calculating the correct charges,” the letter states.