Washington DC - A divided Supreme Court on Tuesday allowed President Donald Trump's ban on transgender military personnel to take effect while litigation proceeds, putting thousands of troops at risk of dismissal.

The decision – which the court's three liberal justices opposed – stayed a preliminary injunction by a lower court that had blocked the implementation of the ban as legal challenges play out.

The ruling is a significant victory for Trump, who has made rolling back transgender rights a major part of his second term in office, and has railed against judges who have blocked parts of his agenda.

In a January 27 executive order, Trump stated that "expressing a false 'gender identity' divergent from an individual's sex cannot satisfy the rigorous standards necessary for military service."

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The Pentagon followed that up with a memo issued in late February stating that it would remove transgender troops from the military unless they obtain a waiver on a case-by-case basis, as well as prevent others from joining.

The Supreme Court's decision to allow the ban to take effect means thousands of currently serving troops could be removed from the ranks.

The restrictions in the Pentagon memo are aimed at those who have been diagnosed with gender dysphoria – of whom there were 4,240 serving in the military as of late last year, according to a senior defense official – as well as those who have a history of the condition or exhibit symptoms of it.

Transgender Americans have faced a roller coaster of changing policies on military service in recent years, with Democratic administrations seeking to permit them to serve openly, while Trump has sought to keep them out of the ranks.

The US military lifted a ban on transgender troops in 2016, during Democrat Barack Obama's second term as president.

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Under that policy, trans troops already serving were permitted to do so openly, and transgender recruits were set to start being accepted by July 1, 2017.

But the first Trump administration postponed that date to 2018 before deciding to reverse the policy entirely.

Trump's restrictions on transgender military service – which underwent changes in response to various legal challenges – eventually came into force in April 2019 following a protracted legal battle that went all the way to the nation's top court.

His Democratic successor, Joe Biden, moved to reverse the restrictions just days after he took office in 2021, but Trump was reelected last year after making clear he would again seek to target transgender rights.