Mattis said he would establish a panel of experts to determine how to implement Trump's order and that now serving transgender troops would be allowed to continue while the study is ongoing.
Trump announced transgender individuals can't enlist in a tweet in June, but it is up to Mattis to decide whether now enlisted transgender troops will be allowed to stay in the military.
They expressed the commonly-held sentiment that anyone who is willing and able to serve in our nation's military should be allowed to do so.
The Pentagon on Friday said that it will allow transgender troops now serving in the US military to re-enlist as it studies the issue.
Since President Trump's initial tweets announcing his intent to break from the open service policy set in place after a comprehensive study including Armed Service leadership, medical experts, and advocacy groups, the Trump White House has been unable to provide any clear justification to this discriminatory policy. But Trump also gave Mattis six months to come up with a policy on how to address those now serving, leaving the door open to permitting their continued service.
"The military's prior considered judgment on this matter should not be disregarded and we should not breach the faith of service members who defend our freedoms, including thosewho, are transgender", he said in a declaration.
Sarah McBride, spokesperson for the Human Rights Campaign, said in a statement the memo and new legislation demonstrate the Pentagon and lawmakers, unlike Trump, are willing to support transgender troops.
He added that while this is the current policy, it only remains in effect until the Pentagon implements its "final policy concerning the military service by transgender individuals".
A Rand study conducted under the Obama administration in 2016 found the costs to be low, and that 18 countries already have transgender service members with few reported impacts on readiness. Many of them have been deployed to warzones multiple times.
Sens. Gillibrand and Collins had initially introduced the legislation as an amendment to the 2018 National Defense Authorization Act (NDAA), but the legislation never made it to a floor vote because of disagreement over debate on the must-pass bill. Kirsten Gillibrand (D-N.Y.) and Susan Collins (R-Maine), the same duo who led the successful bipartisan effort to repeal the military's "Don't Ask, Don't Tell" policy in 2010. Sen. "Any qualified American, regardless of their gender identity, should be able to serve their country".