He gained notoriety as a spy for the German Empire and the actual perpetrator of the act of treason of which Captain Alfred Dreyfus was wrongfully accused and convicted in 1894.

After evidence against Esterhazy was discovered and made public, he was eventually subjected to a closed military trial in 1898, only to be officially found not guilty.

Esterhazy retired from the military with the rank of major in 1898—presumably under pressure—and fled by way of Brussels to the United Kingdom, where he lived in the town of Harpenden in Hertfordshire until his death in 1923.

Growing up under the name of Jean Marie Auguste Walsin, he became a business man and landowner in Nîmes, and added the name of Esterházy, apparently without the agreement of the family, after being acknowledged by his mother about 1797.

In 1892, through the influence of General Félix Gustave Saussier, Esterhazy succeeded in getting a nomination as garrison-major in the Seventy-fourth Regiment of the line at Rouen.

His inheritance squandered, Esterhazy had tried to retrieve his fortune in gambling-houses and on the stock-exchange; hard pressed by his creditors, he had recourse to extreme measures.

Having seconded André Crémieu-Foa in his duel with Édouard Drumont in 1892, Esterhazy claimed that this action had made his family, as well as his military seniors, quarrel with him.

For an officer whose original commission had been irregular, Esterhazy's military advancement had been unusually rapid: lieutenant in 1874, captain in 1880, decorated in 1882, major in 1892.

Convicted, he was cashiered (formally stripped of his military rank in a public ceremony of degradation), and then shipped to the penal colony of Devil's Island (L'île du Diable) off the coast of French Guiana.

[1] In 1896, Lieutenant-Colonel Georges Picquart, the then-new head of the Intelligence Service, uncovered a letter sent by Maximilian von Schwartzkoppen (at the time German military attache to Paris) to Esterhazy.

She published the interviews in September 1898,[8] reporting his confession and writing a leader column accusing the French military of antisemitism and calling for a retrial for Dreyfus.

[9] From Milton Road in the town of Harpenden, Esterhazy continued to write in anti-Semitic papers such as La Libre Parole until his death in 1923.

Doise was not the first writer to explore the hypothesis of Esterhazy as a double agent: earlier writings by Michel de Lombarès and Henri Giscard d'Estaing, though they differed in the details of their theories, also presented this line of argument.

[10] According to Doise, Esterhazy's perceived bitterness and utter lack of patriotic feeling, along with his fluency in German, were qualities that would have helped him to pose as an effective and unrepentant traitor.

[11] The lack of value of the material furnished by Esterhazy soon became so apparent that Alessandro Panizzardi, the Italian military attaché, to whom Schwartzkoppen communicated it without divulging the name of his informant, began to doubt his qualifications as an officer.

[citation needed] The infamous document, or bordereau, which was used to convict Dreyfus had been retrieved from a wastepaper basket at the German Embassy by a cleaning lady who was in the employ of French military counterintelligence.