Defense Secretary Jim Mattis. (Matthias Schrader/Associated Press)

LAST JUNE, the Obama administration took a major step toward affording transgender service members the respect they deserve. After a year-long review, the Pentagon repealed its ban on transgender people openly serving in the military and planned to begin processing transgender recruits this July. But a day before the new rule was to come into effect, Defense Secretary Jim Mattis announced a six-month delay pending further review. This sends a dispiriting message to the estimated 12,800 transgender personnel who currently serve the country.

Before the repeal last summer, many transgender troops were forced to conceal their gender identities to avoid dishonorable discharge. This shroud of secrecy made them less likely to come forward about health issues or report instances of sexual assault. Like the infamous “don’t ask, don’t tell” policy that discriminated against gay men and lesbians in the military until 2011, the ban damaged the safety, mental health and well-being of transgender military personnel. The repeal was overdue, and the final step in the process — allowing transgender recruits to join openly — is still sorely needed to provide transgender personnel with the same privileges as other troops, and to provide the military the services of these recruits.

Mr. Mattis has said that that the postponement “in no way presupposes an outcome.” But questioning the impact of transgender personnel on the “readiness and lethality of the force,” as he did in his memo, undermines the service of thousands of veterans and current service members. It also discourages potential recruits and leaves transgender troops who have come out of the closet in the past year unsure of their future.

Military policy changes on this scale demand study — and such study has been conducted. The Obama administration conducted an extensive review and found that concerns over troop readiness lacked substance . Coupled with precautions the military was planning — such as requiring transgender recruits to have been stable in their new genders for 18 months — the setbacks to the armed forces would likely have been minimal and the benefits large.

If Mr. Mattis and the Joint Chiefs of Staff conduct their six-month review process fairly, it will come to the same conclusion as the previous one: that transgender troops are valued members of the armed forces and deserve to be recognized as such.