The inside story of the case that could set the movement back a generation.One day last December, a clutch of dark-suited lawyers descended the steps of the Supreme Court to a hero’s welcome. The lawyers, from the American Civil Liberties Union, had that morning joined counterparts from the Biden administration in asking the court to block a Tennessee law that bars doctors from providing puberty blockers, cross-sex hormones and surgery to young people who feel that their bodies are the wrong sex. In the plaza outside the court, L.G.B.T.Q. advocacy groups had turned out hundreds of supporters, who hugged, cheered...