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Born in Prague, Czech Republic (then part of the Austro Hungarian empire) Friml showed aptitude for music at an early age. He entered the Prague Conservatory in 1895, where he studied the piano and composition with Antonin Dvorak. Friml was expelled from the conservatory in 1901 for performing without permission.

One of the most popular theatrical forms in the early decades of the 20th century in America was the operetta, and its most famous composer was Irish-born Victor Herbert. It was announced in 1912 that operetta diva Emma Trentini would be starring in a new operetta on Broadway by Herbert with lyricist Otto Harbach entitled The Firefly. Shortly before the writing of the operetta, Trentini appeared in a special performance of Herbert's Naughty Marietta conducted by Herbert himself. When Trentini refused to sing "Italian Street Song" for the encore, an enraged Herbert stormed out of the orchestra pit refusing any further work with Trentini.  Arthur Hammerstein, the operetta's sponsor, frantically began to search for another composer. Not finding any other theatre composer who could compose as well as Herbert, Hammerstein settled on the almost unknown Friml because of his classical training. After a month of work, Friml produced the score for what would be his first theatrical success. After tryouts in Syracuse, New York, The Firefly opened at the Lyric Theatre on December 2, 1912 to a warm reception by both the audience and the critics. The production moved to the Casino Theatre after Christmas, where it ran until March 15, 1913, for a total of 120 performances. After The Firefly, Friml produced three more operettas that each had longer runs than The Firefly, although they are not as enduringly successful. These were High Jinks (1913), Katinka (1915) and You're in Love (1917). He also contributed songs to a musical in 1915 entitled The Peasant Girl.  Trentini was named as a co-respondent in Friml's divorce from his first wife in 1915, and evidence was introduced that they were having an affair. Another show, Sometime, written with Rida Johnson Young and starring Ed Wynn and Mae West, ran well on Broadway in 1918-1919.
Rudolf Friml