Mr. Trump made the declaration on Twitter, saying that American forces could not afford the “tremendous medical costs and disruption” of transgender service members. He said he had consulted generals and military experts, but Jim Mattis, the defense secretary, was given only a day’s notice about the decision…

His decision was announced with such haste that the White House could not answer basic inquiries about how it would be carried out, including what would happen to openly transgender people on active duty. Of eight defense officials interviewed, none could say…

Mr. Mattis, who was on vacation, was silent on the new policy. People close to the defense secretary said he was appalled that Mr. Trump chose to unveil his decision in tweets, in part because of the message they sent to transgender active-duty service members, including those deployed overseas, that they were suddenly no longer welcome.