This article talks about how it is Pence that set Trump to ban trans people in the military. Mattis's February report said they should stay, but Pence changed things up.
https://thinkprogress.org/pence-resp...-f4d3b67bde47/
"When President Trump announced a new ban on transgender people serving in the military late Friday, it was somewhat of a surprise — Defense Secretary Jim Mattis had reportedly recommended in February that Trump allow transgender people to serve. It turns out that Vice President Pence and some of the country’s most prominent anti-LGBTQ activists had a role in reversing the outcome, which explains why the report explaining the decision is rife with anti-trans junk science.
Slate’s Mark Joseph Stern reported Friday night that, according to multiple sources, Pence played “a leading role” in creating the report, along with Ryan T. Anderson of the Heritage Foundation, which has been dubbed “Trump’s favorite think tank,” and Tony Perkins of the Family Research Council (FRC), an anti-LGBTQ hate group. Both Heritage and FRC praised the report Friday. According to Stern’s reporting, it was true that Mattis favored allowing transgender military service, but Pence “effectively overruled” him."