LONDON — In November 2007, a man pleaded guilty in a federal court in Dayton, Ohio, to illicit sexual conduct involving a 13-year-old girl. He was a judo coach, and the girl was a student he had trained closely and brought to international tournaments. Her name was given in court papers simply as “K.H.” or “the victim.”

K.H. was Kayla Harrison, who is now 22. On Thursday she won the first gold medal in judo for the United States. It was a remarkable victory for a woman who had faced far more in her young life than most can fathom and for a sport that is popular worldwide but has remained obscure in the United States.

Of the Americans on the judo team here, Harrison was the favorite, though in the hurried gantlet of matches on Thursday she had to take on a Brazilian who was No. 1 in the world and, in the gold medal bout, a British woman, Gemma Gibbons, who was something of an underdog but still an overwhelming crowd darling. Some opponents in the 78-kilogram class Harrison threw to the ground; others she beat on points when the clock expired.

But there were more American judo victories this week than just Harrison’s. In a lighter weight class, a young doctor’s receptionist named Marti Malloy won a bronze, the second Olympic medal ever for an American woman in judo. A bruiser named Travis Stevens reached the semifinals in his weight class, knocking off the No. 1 seed along the way, and another American, Nick Delpopolo, who won his spot on the team by winning a single do-or-die match against a fellow American, reached the quarterfinals.