One man after another tried to knock out black heavyweight champ Jack Johnson.

But it was a woman from Milwaukee, Belle Schreiber, who dealt him the most devastating blow of his life.

A prostitute who worked at Chicago's most plush bordello when they met in 1909, Schreiber kept company with Johnson - who was almost as famous for his sexual exploits as his title - on and off for two years. During that time, she returned frequently to her native Milwaukee, sometimes on Johnson's arm.

But Schreiber was working in a humbler Washington, D.C., brothel when federal authorities persuaded her to testify against her former lover in what would become a sensational trial in 1913.

Convicted on Schreiber's testimony of violating the 1910 Mann Act against so-called "white slavery," Johnson was sentenced to a year in prison. He fled the country and went into exile for seven years before returning to serve his time. When he got out of prison at 43, his best days in the ring were far behind him.

Why did the woman who had proudly, though not truthfully, called herself "Mrs. Jack Johnson" let herself be used as the establishment's chief weapon against him?

"What her motivations were is a complicated question," historian Geoffrey Ward said in a phone interview from his Manhattan office. "What's fascinating is that there seems to have been real affection between the two of them."

Ward, a longtime Ken Burns collaborator, is the author of the recently published "Unforgivable Blackness: The Rise and Fall of Jack Johnson" (Alfred A. Knopf, $26.95), the companion volume to Burns' PBS film of the same name, which Ward also wrote.

Schreiber, Ward notes, is a somewhat shadowy figure and would certainly be more so if not for U.S. Department of Justice's records from the trial.

She went by many different aliases, as women of her profession often did. She was said to be the daughter of a Milwaukee police officer, a story for which Ward could find no confirmation.

By her own account, the young Belle studied stenography and took dictation from businessmen at Milwaukee's old Plankinton Hotel before turning to prostitution in Chicago. In her early 20s, slender, pretty and dark-haired, she found work at the posh, imposing Everleigh Club.

Black men were not welcome at the Everleigh, but Johnson, then living in Chicago, had connections -- and enough charm -- to get five of the club's girls, all of them white, to break the rules.

Schreiber went to see Johnson in his rooms and, for this offense, was fired by the Everleigh. So she went on the road with him, a favorite mistress who occasionally posed as the fighter's wife.

Favorite did not mean only. But Schreiber appears to have taken the competition in stride until Johnson did the unthinkable with still another white girlfriend, Etta Duryea, in 1911: He married her.

When news of the marriage broke, there was a huge scandal: She was a woman from a "good" family, and he, no matter his title, was nothing more than a "giant black," to use one of the more polite descriptions in the newspapers of the day.

The buzzing over the marriage, however, was nothing compared to the shock that went through white society when Etta Duryea Johnson committed suicide the following year. To make matters worse, just months after that tragedy, Johnson announced his intention to take another white bride, 19-year-old Lucille Cameron.

His marrying white women, however outrageous it was considered at the time, was not illegal. But prosecutors believed they had a case against him based on the then-new Mann Act, which made it a federal offense to transport a woman across state lines "for the purpose of prostitution or debauchery, or for any other immoral purposes."

When Lucille refused to cooperate with the authorities, they cast their net wider and pulled in Belle. This time, they had their woman.

Stung by Johnson's marriage to Etta, then by his attachment to Lucille, Schreiber had another reason for resenting him. The notoriety she had won as his mistress not only cost her the Everleigh job but later got her banned from "respectable" houses of prostitution.

By the time she turned up in Washington, Ward writes, Schreiber was hooked on absinthe and narcotics. She became easy prey for officials.

Sequestered before the trial in New York City, the increasingly erratic Belle complained bitterly of her imprisonment, then accosted a fellow passenger as she was being taken by train back to Washington.

On the stand, however, she was modest and restrained, Ward writes. She testified that she had traveled extensively with Johnson and accepted money for sex; that he had paid for her to move from Pittsburgh to Chicago, where he set her up in an apartment as a madam and prostitute; that they had traveled several times between Milwaukee and Chicago, crossing still another state line "for immoral purposes"; and that Johnson regularly beat her.

Johnson adamantly denied the beatings. As for the rest of the allegations, his attorneys noted that Schreiber, far from being enticed into a life of sin by Johnson, had been a prostitute when he met her. The nearly $2,000 he spent on her Chicago flat, they maintained, was not the investment of a procurer but the gift of a friend.

But his defense didn't impress the jury. It didn't help, either, that a defiant Johnson had married Lucille Cameron before the trial began.

Johnson lived for another 33 years after his conviction. Lucille divorced him in 1924, but he found enduring love with his third and final wife, Irene Marie Pineau.

What became of Schreiber?

"After the trial, there's nothing" by which to trace her, Ward says. That isn't unusual for a woman of her station: In fact, the remarkable thing is that her testimony against Johnson revealed so many details about her own life.

The detail that fascinates Ward is that, for all the bitterness she bore Johnson, she couldn't renounce her affection for him, perhaps even her love.

Cross-examining Belle, Ward writes in his book, defense attorney Benjamin Bachrach showed her the photograph Jack had inscribed to her and asked: "Were you his little sweetheart'?"

Schreiber: "I suppose I was."

Bachrach: "Were you in love with him?"

Schreiber: "I don't know. ... I don't know what love is."

Pressed further by Bachrach -- "Did you love him then ...? Do you think you loved him then?" -- Belle would only say: "I don't remember what I thought. My memory is good on all the other portions of the testimony, but about this one question -- whether I was in love with him or not -- my memory is poor."

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