trump first announced a policy banning transgender people from serving in the military in "any capacity" in a series of tweets on july 26, 2017, stating that allowing such service members would incur "tremendous medical costs and disruption". the decision reversed obama administration policy to allow the enlistment of transgender personnel, which was initially approved by the department of defense to begin july 1, but was delayed by defense secretary mattis. trump issued formal guidance on the ban to the secretaries of defense and homeland security in a memorandum on august 25, 2017.