Esperanto was developed in the late 1870s and early 1880s by ophthalmologist Dr. Ludovic Lazarus Zamenhof, an Ashkenazi Jew from Bialystok, now in Poland and previously in the Polish-Lithuanian Commonwealth, but at the time part of the Russian Empire.
After some ten years of development, which Zamenhof spent translating literature into the language as well as writing original prose and verse, the first book of Esperanto grammar was published in Warsaw in July 1887. The number of speakers grew rapidly over the next few decades, at first primarily in the Russian empire and Eastern Europe, then in Western Europe, the Americas, China, and Japan. In the early years, speakers of Esperanto kept in contact primarily through correspondence and periodicals, but in 1905 the first world congress of Esperanto speakers was held in Boulogne-sur-Mer, France. Since then world congresses have been held in different countries every year, except during the two World Wars. Since the Second World War, they have been attended by an average of over 2000 and up to 6000 people.
As a potential vehicle for international understanding, Esperanto attracted the suspicion of many totalitarian states. The situation was especially pronounced in Nazi Germany and in the Soviet Union under Joseph Stalin.
In Germany, there was additional motivation to persecute Esperanto because Zamenhof was a Jew. In his work Mein Kampf, Hitler mentioned Esperanto as an example of a language that would be used by an International Jewish Conspiracy once they achieved world domination. Esperantists were executed during the Holocaust, with Zamenhof's family in particular singled out for execution.
In the early years of the Soviet Union, Esperanto was given a measure of government support, and an officially recognized Soviet Esperanto Association came into being. However, in 1937, Stalin reversed this policy. He denounced Esperanto as "the language of spies" and had Esperantists executed. The use of Esperanto remained illegal until 1956.