It's already been halted by the courts pending trial.

From the clickable link below:

U.S. judge in Seattle has ordered President Donald Trump not to ban transgender troops from serving in the military, saying it's unclear whether recent changes to his administration's policy are constitutional.

U.S. District Judge Marsha Pechman was one of four judges nationwide who blocked Trump late last year from overturning an Obama-era directive allowing transgender troops to serve openly. The Justice Department asked her to dissolve that ruling, citing changes that would allow transgender troops to serve in limited cases.

Because the changes were announced just last month, the sides had not had time to argue whether the policy is discriminatory or whether the military is entitled to set its own policy, given its expertise in what's needed for national defense, Pechman said in an order Friday. She told the parties to prepare for trial.

"Because transgender people have long been subjected to systemic oppression and forced to live in silence, they are a protected class," Pechman wrote. "Therefore, any attempt to exclude them from military service will be looked at with the highest level of care."

Under Pechman's ruling, that policy can't take effect. She said the changes didn't truly amount to a new policy but "instead threaten the very same violations" that led her and other judges to block the ban.

Her ruling came in a challenge brought by several transgender troops or prospective troops, as well as civil rights organizations including Lambda Legal. The state of Washington is also a plaintiff.

"The court's ruling means President Trump's hastily conceived and discriminatory ban now has to stand up to the court's most rigorous standard of review," Attorney General Bob Ferguson said in a statement Sunday night. "I am confident the ban will not withstand that."

Also see "Trump’s Trans Troops Ban Will Never Take Effect" at:

https://slate.com/news-and-politics/2018/03/trumps-new-trans-troops-ban-is-still-unconstitutional.html