Around 9000 people, who self-identify as transgender individuals, currently serve in armed forces.
Under President Donald Trump's new rules, which will go into effect April 12, only transgender troops serving in their birth gender will be able to stay in the military, and will be barred from taking hormones or getting transition surgery.
The move was a reversal from a policy enacted under Trump's Democratic predecessor Barack Obama, who allowed military recruits to serve in their preferred sex, rather than just the one they were assigned at birth.
The latest action has been swiftly condemned by Democrats and civil rights advocates.
Protesters gathered in Times Square in 2017 to protest Trump's announcement. Source: APP
"The Trump administration is determined to bring back 'don't ask, don't tell,' a policy that forced service members to choose between serving their country and telling the truth about who they were," said Aaron Belkin, director of the Palm Center, an independent research institute.
Trump announced the ban by tweet in July 2017, pointing to the "tremendous medical costs and disruption that transgender in the military would entail".
After a flurry of legal battles, the Pentagon's policy is not a complete ban.
Transgender troops have until next month's deadline to receive hormonal treatment or surgery, and service members who have already joined the military in their preferred gender will be able to continue to serve in that gender.
After April 12, however, no one who has transitioned to another gender or been diagnosed with "gender dysphoria" will be able to enlist, and currently-enlisted troops will thereafter have to serve in their birth gender and renounce transition treatment.
It will be required for most individuals to serve in their birth sex.
Around 9000 people who self-identify as transgender individuals currently serve in the armed forces, out of 1.3 million in the active duty troops population, a Pentagon official told AFP.
Members of the US military. Source: APP
Around 1000 say they have transitioned to another gender or would like to do so.
"Persons with a history of gender dysphoria - a serious medical condition - and who have undergone certain medical treatment for gender dysphoria, such as cross-sex hormone therapy or sex reassignment surgery, or are unwilling or unable to meet the standards associated with their biological sex, could adversely impact unit readiness and combat effectiveness," a Pentagon statement read.
"For this reason, such persons are presumptively disqualified for service without a waiver."
