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The directive gives the Department of Defense six months to develop an implementation plan that will go into effect on March 23, 2018.
The overall basis for the president's directive was "national security considerations," according to a senior White House official.
"In my judgment, the previous administration failed to identify a sufficient basis to conclude that terminating the departments' long-standing policy and practice would not hinder military effectiveness and lethality, disrupt unit cohesion, or tax military resources, and there remain meaningful concerns that further study is needed to ensure that continued implementation of last year's policy change would not have those negative effects," Trump's memo reads.
The memo comes one month after Trump said he would not permit transgender individuals from serving, tweeting "the military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail."
Trump's guidance effectively returns the Pentagon to its policy before June 2016, when then–Defense Secretary Ash Carter allowed transgender individuals to serve openly, permitting the funding of treatments and gender reassignment surgeries.
Carter gave one year for the Pentagon to study how to allow transgender individuals to join the military — referred to as accession.
This past June, Secretary of Defense James Mattis extended that study through January 2018. The new White House memo will extend the ban indefinitely until "such time that the defense secretary recommends against the contrary," the official said.
The Department of Defense is also directed to stop all gender-related surgeries, with the exception of those individuals whose procedures are already underway — to protect the health of the individual.
As for transgender individuals currently serving, the Pentagon will have six months to formulate a policy for how to handle their service.
The official would not outline the “factors” that Mattis could use to come up with that policy, leaving open the possibility that some transgender service members could keep their jobs.
"The Department of Defense has received formal guidance from the White House in reference to transgender personnel serving in the military. More information will be forth coming," said Dana White, assistant to the secretary of defense for public affairs, in a statement on Friday.
A 2016 Rand Study, commissioned by the Department of Defense and cited by Carter last summer, looked at the effects of integration efforts of foreign militaries and determined "little or no impact on unit cohesion, operational effectiveness, or readiness."
"Policy changes to open more roles to women and to allow gay and lesbian personnel to serve openly in the U.S. military have similarly had no significant effect on unit cohesion, operational effectiveness, or readiness," Rand said.
When asked how much had been spent on transgender-related medical procedures over the last year, the official referred questions to the Pentagon.
The Rand study estimated extending gender transition–related health care coverage to active duty transgender service members would increase health care costs "by between $2.4 million and $8.4 million annually, representing a 0.04- to 0.13-percent increase in active-component health care expenditures."