Answer by taking a quote from the following article:

Evita is a musical with music by Andrew Lloyd Webber and lyrics by Tim Rice. It concentrates on the life of Argentine political leader Eva Peron, the second wife of Argentine president Juan Peron. The story follows Evita's early life, rise to power, charity work, and eventual death. The musical began as a rock opera concept album released in 1976.

In 1972, Robert Stigwood proposed that Andrew Lloyd Webber and Tim Rice develop a new musical version of Peter Pan, but they abandoned the project.  Travelling late to a meal one night in 1973, though, Rice heard the end of a radio show about Eva Peron which intrigued him. As a child stamp collector, he had been fascinated by her image on the Argentine stamps, but was unaware of her significance in Argentina's history. He began research and was introduced by a Cinema International Corporation executive to the Argentine film director Carlos Pasini Hansen who had produced the TV film Queen of Hearts, which had aired in the UK on 24 October 1972. The executive also arranged for Rice to see the film at Thames Television which he did "at least twenty times" saying also that "by that time I had seen Pasini's superbly researched film, I was hooked." The more Rice investigated Eva Peron, going so far as to travel to Buenos Aires to research her life with many documents and contacts that Pasini had supplied, the more fascinated he became by the woman; he even named his first daughter after her.  Rice suggested the idea of a musical based on the subject to Lloyd Webber, but although the idea of writing a score including tangos, pasos dobles, and similar Latin flavours intrigued him, Lloyd Webber ultimately rejected the idea. He decided instead to collaborate with Alan Ayckbourn on Jeeves, a traditional Rodgers and Hart-style musical based on the P. G. Wodehouse character, which proved to be a critical and commercial failure. After Jeeves, Lloyd Webber returned to Rice, and they began developing Rice's proposed musical.  The authors of the 1996 book Evita: The Real Life of Eva Peron claim the musical was based on Mary Main's biography The Woman with the Whip, which was extremely critical of Eva Peron. Though Rice praised the Main biography, it was never officially credited as source material. Rice created a character, Che, to serve as a narrator and Greek chorus. Although he had recently discovered Che Guevara was Argentine, he did not necessarily intend that the character be based upon him, despite inserting specific biographical details into the lyrics that clearly apply to Guevara. When Harold Prince later became involved with the project, he insisted that the actors portraying Che should use Guevara as a role model. In the 1996 film adaptation, the character returned to his more anonymous roots. This was also the case for the 2006 London revival.  Lloyd Webber and the conductor Anthony Bowles presented the musical at the second Sydmonton Festival before making the recording with the London Philharmonic Orchestra.

What was the history behind it?
inserting specific biographical details into the lyrics that clearly apply to