In September 96, Emperor Domitian was succeeded by Marcus Cocceius Nerva. After a brief and turbulent year in power, culminating in a revolt by members of the Praetorian Guard, Nerva was compelled to adopt the more popular Trajan as his heir and successor.
Since Nerva was really unpopular with the army and had recently been forced to execute Domitian´s killers by his Praetorian Prefect, he felt the need to gain support of the military in order to avoid being deposed. He accomplished this on October 28 of 97 by naming Trajan as his adoptive son and successor, pleading only Trajan´s outstanding military merits. There are hints that Trajan´s adoption was imposed on Nerva, as Pliny wrote, and if this is what happened, then Trajan would be an usurper, and the notion of natural continuity between Nerva and Trajan´s reigns would be a fiction created by latter historians such as Tacitus.