The administration said there is "too great a risk to military effectiveness and lethality" to allow transgender people to serve openly - a policy enacted under Trump's predecessor Barack Obama. The court decided by a narrow 5-4 majority to suspend prohibitions restricting transgender service as litigation moves forward in lower courts.
Under the Obama-era policy, transgender recruits were to start being accepted by July 1, 2017. The Trump administration postponed that date to January 1, 2018, before deciding to reverse the policy entirely. But the ban on transgender people in the military was repeatedly challenged in court, leading to an updated policy that also contained major restrictions on transgender service and which was also suspended because of its similarity to the original measure.
Transgender troops have been serving openly since January 1, 2018, but the government appealed and asked the Supreme Court to hear the case, requesting that it suspend the rulings of the lower courts. The number of transgender troops among America's approximately 1.3 million active duty service members is fairly small, with estimates topping out at 15,000.