Excerpts from Books and Wikipedia
"On January 27, 1186, Henry, the son of the Hohenstaufen Holy Roman Emperor Frederick Barbarossa, married Constance of Sicily. . . . Henry was heir to the Holy Roman Empire, which included northern Italy. Constance was the aunt and heir of the childless Norman king William II of Sicily, whose kingdom also included most of southern Italy. A union between the Holy Roman Empire and the Kingdom of Sicily was not welcomed by the pope. It had been less than a century since Pope Gregory VII and Holy Roman Emperor Henry IV had struggled for control over the power to appoint church officials in what is known as the Investiture Contest. . . . The prospect of German rule was not welcomed in Sicily either. When William II died, the Sicilians chose as their king Tancred of Lecce, the illegitimate son of Constance's brother Roger. It was not until Tancred's death in 1194 that Henry and Constance won control of Sicily." [Furtado: 1001 Days]