Ferdinand Walsin Esterhazy

Charles Marie Ferdinand Walsin Esterhazy (16 December 1847 – 21 May 1923) was a commissioned officer in the French armed forces during the second half of the 19th century who has 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 (see Dreyfus affair).

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. A revisionist theory raises the possibility that Esterhazy may have been a double agent working for the French counter-espionage service and that this could help to explain the degree of protection he received. (See section below.) This thesis has not gained general acceptance, the consensus being that the high command saw its own credibility as bound up with upholding the earlier conviction of Dreyfus.

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 village of Harpenden in Hertfordshire until his death in 1923.