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.

Mr. Trump elected to announce the ban in order to resolve a quietly brewing fight on Capitol Hill over whether taxpayer money should pay for gender transition and hormone therapy for transgender service members. The dispute had threatened to kill a $790 billion defense and security spending package scheduled for a vote this week.

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