The Protocols of the Elders of Zion
In 1903, in Russia, a book was published claiming to be minutes of a late 19th-century meeting where Jewish leaders discussed their goal of global Jewish hegemony by controlling the press and the world's economies. Its intent was to portray Jews as conspirators against the (Russian) state. Within a few years, the book was translated into various European languages and published in Europe, and by 1920 1/2 a million copies of the Protocols of the Elders of Zion were published in the USA by motor-car magnate Henry Ford and serialised in a newspaper he owned. Ford cited the text as evidence of a Jewish threat to the USA. By 1921 it was pretty well accepted, at least in academia, that they were a completely fabricated forgery, one which had substantially plagiarized various other (non anti-Semitic) texts such as French writer Maurice Joly's 'Dialogue in Hell', published in the mid-1800's as a fictional critique of Napoleon III.
Of course, the text was widely disseminated in Europe in the early 20th C and were referenced by Hitler in 'Mein Kampf'. The text was read out in Nazi rallies and meetings, and it was disseminated in schools as a literary text in Germany in this period. Though largely discredited in mainstream politics in the post-war period, they are still accepted as authentic by some leaders in Arab and Muslim regimes, and by anti-semites around the world.
It is hard to quantify the damage this single piece of fake news has done, but Holocaust historian Nora Levin wrote: "They were translated into every language of Europe and sold widely in Arab lands, the US, and England. But it was in Germany after World War I that they had their greatest success. There they were used to explain all of the disasters that had befallen the country: the defeat in the war, the hunger, the destructive inflation". Anti-Semitism has been rife in Europe for centuries but the publication of this text gave voice to the fiction that Zionists (i.e. Jews) were secretly collaborating to dominate the world by controlling the banks, the media and industry. Unfortunately despite their obvious lies and overt anti-Semitic hatred, the text is still published both in print and online. But, perhaps more dangerously, the ideas it spawned of a grand Zionist/Jewish conspiracy to dominate media, finance and industry are accepted by many people as fact - at the very least as a racist Jewish caricature - even by people who would never think of themselves as being anti-Semitic.