Complex sentence: Otho had governed Lusitania, and was one of Galba's earliest supporters, disappointed at the selection of Piso, entered into communication with the discontented Praetorians, and was hailed by them as their emperor on 15 January 69.
Split into simpler sentences: Otho had governed Lusitania and was one of Galba's earliest supporters. He was disappointed at the selection of Piso and entered into communication with the discontented Praetorians, and he was hailed by them as their emperor on 15 January 69.