Dreyfus’s imprisonment gave rise to a twelve year controversy, that divided the French, largely on the issue of Antisemitism.
Dreyfus was accused of selling military secrets to the German military attaché while assigned to the war ministry, due to a letter that was found. Despite claiming his innocence, he was convicted of treason on December 22 and sentenced to life imprisonment. However, the evidence against him was highly irregular and the trial largely based on Anti-Semitic rhetoric.
Doubts on Dreyfus’s involvement soon arose, when Lieutenant Colonel Georges Picquart found evidence of espionage on account of a Major Ferdinand Walsin-Esterhazy (his handwriting matched that of the incriminating letter). However, his superiors removed Picquart from his post and police subsequently arrested him.

Esterhazy continued to spread rumors and invent evidence with the help of Major Hubert Joseph Henry, discoverer of the original letter attributed to Dreyfus, who continued to forge new documents and suppress others.

Nevertheless, support for Dreyfus’s innocence grew. In early 1898, novelist Émile Zola wrote an open letter entitled “J’Accuse”, accusing the military of covering up the wrongful conviction, which was published and widely read across the country. Zola was persecuted by anti-Semitic nationalists, and subsequently convicted of libel and imprisoned.

A real break in the case came after Major Henry confessed to his forgeries (before committing suicide). Esterhazy fled the country and Dreyfus’s was granted a retrial.

Despite being found guilty again in 1899 by the a new-court martial, Dreyfus was pardoned by the president of the republic in an effort to resolve the issue. Efforts to clear Dreyfus’s name however continued .

In 1906, a civilian court of appeals cleared Dreyfus and reversed all previous convictions. He was formally reinstated and decorated with the Legion of Honour, and later saw active service in World War I.

The affair largely marked the history of the French Third Republic. The turmoil resulted in the separation of church and state in 1905 and brought forth a passionate repudiation of anti-Semitism.

The army did not publicly declare Dreyfus’s innocence until 1995.