Defense Secretary Jim Mattis at the White House on Dec. 12, 2017. (Salwan Georges/The Washington Post)

AS OF Jan. 1, the United States military finally began accepting transgender recruits. That's despite President Trump's tweets over the summer demanding a ban on all military service by transgender Americans. Barred multiple times by federal judges from implementing the president's order, the Trump administration chose to hold back from further efforts to prevent new recruits from joining — at least for the moment.

Under an order by President Barack Obama's defense secretary, Ashton B. Carter, the Pentagon was set to begin accepting transgender recruits as of July 2017. But Mr. Trump's defense secretary, Jim Mattis, had already pushed that deadline to January 2018 when the president demanded that transgender enlistment be called off entirely. Mr. Trump's memorandum also requested that Mr. Mattis issue a report by Feb. 21, 2018, on the fate of transgender personnel currently serving. The defense secretary has promised that no service members will be discharged for their gender identity in the interim.

The new policy has been barred from going into effect since late October, when the U.S. District Court for the District of Columbia found that Mr. Trump's order likely violated constitutional prohibitions against discrimination on the basis of gender identity. Since then, three more federal district courts have reached the same conclusion. Their injunctions prevented the military from expelling transgender service members and required it to begin welcoming transgender recruits in the new year, as originally intended.

The Justice Department appealed three of these cases to higher courts, arguing that the government should be allowed to delay transgender people from signing up for service while litigation progressed. Two appeals courts rejected their arguments. Then, after beginning another round of appeals, the government decided instead to pause its defense of Mr. Trump's order while awaiting the Pentagon's Feb. 21 report.

During Mr. Trump's first year in office, the Justice Department regularly turned to the Supreme Court to fight lower-court injunctions. This choice to yield gracefully — however briefly — is a welcome shift. Already, transgender Americans seeking to serve their country have begun applying to join the military.

Yet those new recruits face a great deal of uncertainty as Mr. Mattis prepares his report to the president. While the litigation over Mr. Trump's policy will likely continue, it's not clear what course the Pentagon will recommend regarding recruitment or transgender people already in the armed services.

The good news is that the Defense Department already knows that allowing military service by openly transgender personnel would have a great number of benefits and negligible costs. A 2016 study by the Rand Corp. found just that — one more reason Mr. Trump's ban was nonsensical. If Mr. Mattis's February review is fair, he will find that there's no reason to prevent transgender men and women from joining and serving. The president's order has thrown the lives of many current and would-be service members into needless turmoil because of their gender identity. The Pentagon should take this opportunity to set things right.