How did Strauß come to be a composer of operettas ?

On March 17th, 1865 Jacques Offenbach, the creator of the French operetta, conducted his "Die schöne Helena“ at the Theater an der Wien, with Marie Geistinger as the lead singer. Offenbach was the unsurpassed master of the operetta market in Vienna, but made a remark to Strauß, inviting him to try his hand at this genre. 
A further step for Strauß towards developing an interest in this genre was a private performance at the empty Theater an der Wien for which friends of Strauß had arranged melodies from Strauß scores (pillaged by Jetty) with added texts and instrumentation. Strauß enjoyed this newly arranged music - thus the initiative of his wife directed him on the way to become the immortal King of waltz and operettas. On October 10th, 1871 his first operetta "Indigo und die 40 Räuber“ ("Indigo and the 40 thieves“) was performed at the Theater an der Wien. 

Jacques Offenbach and Johann Strauß