In 1887 Polish linguist and ophthalmologist Ludwik Zamenhof published his constructed language Esperanto. Zamenhof had grown up in Bialystok, a multi ethnic city sharply divided along religious, ethnic, and linguistic lines. Polish, Russian, German, and Yiddish speakers lived together in a state of mutual distrust.
It was in this environment that Zamenhof came to the idea of a universal auxiliary language. The purpose of this language would not be to displace any existing language, rather it would be a second, easy to learn, language that everyone could use to make themselves understood. Since it was a constructed language it would not carry the baggage of being the language of a dominant group. It would not be imposed by some conqueror, but would be freely accepted as a convenient second language.
The other advantage of a constructed language is that it could be structured in such a way as to make it easy to learn. Thus, Esperanto has no irregular verbs, its pronunciation is standard, and it has many other features that simplify the learning process. For example, any word can be made into its opposite by simply adding the prefix “mal.” So the word for big is "granda," and the word for small is "malgranda." This simple principle greatly reduces the amount of vocabulary that has to be learned.
Esperanto has experienced something of a revival. The Internet has allowed for increased communication between Esperantists and more opportunities to learn the language.
There have been many efforts to create universal auxiliary languages, some of them attempts to improve on Esperanto. However, Esperanto has one virtue over these other efforts, it already has a large number of speakers and has more momentum.