In June 2016, then-Defense Secretary Ashton Carter lifted the Pentagon's long-standing ban on transgender people serving openly in the US military. Medical costs for gender-affirming surgery were to be covered by the Pentagon for uniformed military personnel. 

In July 2017, President Trump tweeted that no transgender people would be allowed "to serve in any capacity in the U.S. Military." The policy change went through several courts before landing at the US Supreme Court where, in a 5-4 ruling, the ban was allowed to stand.

The ban, which went into effect on Apr. 12, 2019, includes a prohibition against transitioning for transgender troops and recruits, a requirement that most serve as their birth gender, and allows a service member to be "discharged based on a diagnosis of gender dysphoria." Gender dysphoria is when a person is distressed due to an inconsistency between their biological sex and their gender identity.

Source: Orion Rumbler, "Everything You Need to Know about the Transgender Military Ban," axios.com, Apr. 12, 2019