US President Donald Trump's controversial ban on transgender Americans in the military came into force on Friday following a protracted legal battle that went all the way to the nation's top court.
Trump's administration has insisted that there is "too great a risk to military effectiveness and lethality" to allow transgender people to serve -- reversing a policy enacted under his predecessor Barack Obama.
The Pentagon says the restrictions are not a blanket ban, but they would bar many if not most people who identify as transgender from enlisting, and could also impact currently serving personnel.
"This policy will ensure that the US military maintains the highest standards necessary to achieve maximum readiness, deployability, and lethality to fight and win on the battlefield," the Department of Defense says on its website.
The policy -- which has undergone various iterations since Trump first announced it on Twitter in July 2017 -- has been widely criticized by rights activists and has been repeatedly challenged in court.
The US Supreme Court ruled in January that the policy could take effect pending the outcome of ongoing litigation.
Under the latest version, no one who has transitioned to another gender or who requires hormone treatment will be able to enlist.
Those who have been diagnosed with "gender dysphoria" are also "presumptively disqualified" from enlisting unless they have been "stable for 36 months and (are) willing and able to serve in (their) biological sex," according to the Department of Defense.
Gender dysphoria, according to the American Psychiatric Association, "involves a conflict between a person's physical or assigned gender and the gender with which he/she/they identify."
Serving troops who transitioned or requested gender reassignment surgery before Friday are allowed to remain in the military.
But starting on Friday, those who are newly diagnosed with gender dysphoria may be discharged if they are "unable or unwilling to serve in (their) biological sex," the Department of Defense says.
- Repeating 'don't ask, don't tell' -
Aaron Belkin, director of the Palm Center, a research institute focusing on sexual minorities in the military, said the policy amounts to a ban that will force transgender troops to remain "silent and invisible."
"It is 'don't ask, don't tell' all over again," said Belkin, referring to the policy under which gay service members had to hide their sexual orientation or face dismissal from the military.
Sarah McBride of the Human Rights Campaign, which advocates for the LGBT community, described the policy as a national security threat.
"The Trump-Pence transgender troop ban is dangerous for both transgender people and our national security," McBride said in a statement.
"The fact that a service member who came out on Thursday can continue to serve openly while a service member who comes out on Monday can't only reinforces the cruel and arbitrary nature of this ban," she said.
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.
The Pentagon estimates that 9,000 out of a total of 1.3 million active-duty personnel identify as transgender. Of those, 1,000 say they have undergone gender reassignment surgery or want to.
Transgender rights activists say the figure is higher.
"As many as 15,000 transgender service members stand to lose their jobs," Army Staff Sergeant Patricia King, who is transgender, told ABC News this week.
"For those of us who are grandfathered in because we've already come out and we already have a diagnosis, there's the possibility for systematic discrimination," King said.
Former secretary of state Hillary Clinton, who ran against Trump in 2016, described the new rules as "abhorrent."
"Every American should be able to serve their country, and I can't believe we're still debating that in 2019," she tweeted.
"We have to reverse this as soon as we can."