The lawsuit by the American Civil Liberties Union, filed Monday in federal court in Maryland, alleges Trump's order violates the Equal Protection clause of the Constitution.
In addition, a group of current and former military professors released a memo Monday morning arguing that the President has left the secretaries of Defense and Homeland Security little discretion in addressing the status of current transgender persons. It's unknown how many of them are in Georgia, which is home to thousands of USA service members and some major military installations, including Fort Benning, Fort Gordon and Fort Stewart.
The memo indicates a reversal of an Obama-era policy implemented in 2016, which allowed active-duty service members who are transgender to serve openly and transition while enlisted.
On a statement on the ACLU website, Joe Block, a senior staff lawyer for the ACLU's LGBT & HIV Project said: "Each and every claim made by President Trump to justify this ban can be easily debunked by the conclusions drawn from the Department of Defence's own review process".
Each lawsuit challenges the president's assertions that transgender people are a burden on the military due to their medical costs and that their service hurts the military's "readiness", citing studies finding that the costs are "minimal" and that in the 18 countries that now allow transgender people to serve openly, "there has been no significant effect" on readiness or cohesion.
Trump announced the formal start of the process to ban transgender individuals from the military Friday night, as Hurricane Harvey bore down on the Texas coast.
"After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow ...", he wrote on Twitter.
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Estimates of the total number of transgender troops in the US military range from just over 1,000 to more than 10,000. Our military must be focused on decisive and overwhelming ... victory and can not be burdened with the tremendous medical costs and disruption that transgender in the military would entail.
In tweets announcing the ban, Trump stated that "our military must be focused on decisive and overwhelming victory and can not be burdened with the tremendous costs and disruption that transgender in the military would entail".
The Pentagon now permits transgender individuals already serving in the armed forces to do so openly, a policy enacted in June 2016 under the Obama administration. If he decides now serving transgender troops are not causing problems and should be allowed to continue to serve, does that provide the proof that Trump is asking for?
The 39-page ACLU suit, Stone v. Trump, is the third such court action in response to Trump's decision.
The ACLU's complaint purports to address each of Trump's stated concerns, using government-backed studies, an analysis of transgender-related costs and the apparent readiness of several US allies that allow transgender soldiers to serve openly.
OutServe-SLDN, a nonprofit group that advocates for transgender people, immediately announced it would sue to challenge Trump's directive. The authors of the study concluded that allowing transgender personnel to serve openly will have "minimal impact on readiness".