Just to add to Turnbull's quotation, Tom never took for granted that he would obtain the position. He was "interim" while Genco was still alive (I think for approximiately 1 year), but was definitely permanent when Genco died.

All of the Corleone enemies, including Sollozzo, held little account for Hagen. In the novel, Sollozzo said something to the effect (when he snatched Hagen) "you're not even Italian, much less Sicilian." Finally, at the meeting of the Families to negotiate the peace, Puzo remarked how some of the Families referred to the Corleone's as the "Irish Gang" because of Hagen's Irish heritage.