Publius Aelius Hadrianus was born on January 24, AD 76 likely in Rome but possibly in the same place of Trajan's birth, Italica, Hispania Baetica. Regardless, Hadrian certainly spent much of his youth in Rome where his father, P. Aelius Hadrianus Afer, had served various magistracies including the praetorship. His family, though probably considered obscure and provincial by imperial standards (it had moved from Picenum in Italia to Hispania shortly after the Second Punic War some three centuries prior), was to gain rapid advancement along with the rise of Trajan in political and military significance. Trajan was the cousin to Hadrian's father and with the father's death in about AD 86 Trajan became joint guardian of the young future emperor along with P. Acilius Attianus. As he and his wife Pompeia Plotina had no children of their own, they certainly helped fill a role as surrogate parents, though there was no official adoption.
Hadrian was in Rome as a teenager and with the backing of Trajan, one of Domitian's key legionary commanders, he followed a rapid and customary rise through the cursus honorum. By the age of 20 Hadrian was serving his first of three consecutive military tribunates, perhaps indicating Trajan's desire for his surrogate son to follow his military footsteps. With the assassination of Domitian in AD 96, the accession of Nerva and the adoption of Trajan as his heir (because of his position of importance as a leading general able to quell any threat of a legionary mutiny against the new emperor) Hadrian quickly found himself thrust into the heart of imperial politics. It was Hadrian, while serving his second military tribunate in Moesia, who personally carried the congratulations and loyalty of the Danubian legions to Trajan. Having arrived and affirmed his place of importance in Trajan's court, Hadrian was kept on hand, appointed to a third tribunate of one of Trajan's own legions, Legio XXII Primigenia in Germania Superior.
Within a year of Trajan's adoption as heir to the Roman 'throne', Nerva died (AD 98) and Hadrian's influence in imperial affairs continued to rise along with the new emperor. By AD 100, Pompeia Plotina who remained an influential and driving force in Hadrian's career arranged a marriage with Trajan's grand niece Vibia Sabina. Though Hadrian would remain married until his wife's death in AD 136 or 37 the marriage was one of complete unhappiness. Despite the unpleasant nature of his marital relationship, Hadrian remained close to Pompeia and also to his mother-in-law Matidia (Trajan's niece). Though their favor, and certainly his own merits, Hadrian continued to advanced during Trajan's principate.
He accompanied the emperor as a quaestor on his first campaign in Dacia, commanded a Legion (I Minervia) during the second war (AD 105-6) and held several posts of military importance throughout Trajan's reign. Shortly after Dacia, he was sent to govern Pannonia where a stern hand was needed to discipline Roman troops and to fend off incursions by Sarmartian neighbors. As a reward for exemplary services, Hadrian was awarded his first consulship in AD 108 and continued to serve various official roles over the course of the next several years. According to the questionable ancient source, the Historia Augusta, a steady flow of honors continued to be awarded to Hadrian until his ultimate adoption as Trajan's heir. When Trajan began his campaign against the Parthians (circa AD 114), Hadrian was eventually appointed to a key eastern command. With Hadrian serving as governor of Syria in AD 117 Trajan's failing health forced him to abandon the war, leaving Hadrian in command of the east. While en route to Rome, Trajan's health worsened and he died on August 8 at Selinus in Cilicia.
The circumstances surrounding Hadrian's adoption and ultimate accession have long been clouded in mystery. Though there were clear indications as to Hadrian's favored status as a member of the emperor's inner circle, Trajan never made a public endorsement of him, or any other potential candidate as heir. The highly dubious account recorded in the Historia Augusta even suggests that Trajan purposely did not leave an heir in order to follow the example set by Alexander the Great, leaving the matter either to be decided by the Senate or his various generals. Dio Cassius reported that Trajan's widow Pompeia actually secured Hadrian's adoption, announcing the adoption posthumously through letters signed in her own hand rather than Trajan's. Regardless of the truth of the matter, the choice proved a wise one in that it certainly helped prevent the potential for civil war. Hadrian was announced as Trajan's heir by Pompeia and by the Praetorian Prefect Attianus (the man appointed as joint guardian of the youthful Hadrian along with Trajan) on August 9, AD 117, though the emperor's death was still kept secret from the public. Two days later, on the 11th, when Trajan's death was finally declared, the Syrian legions hailed Hadrian as the new emperor and the matter was reduced to a mere formality awaiting Senatorial approval.
Hadrian traveled immediately to Selinus where he was present for Trajan's cremation and likely confirmed the loyalty of key members of his predecessor's entourage. Rather than immediately travel to Rome, however, Hadrian returned to Syria where he seems to have spent he better part of the next year preparing his administration and settling eastern affairs. Despite the support of his legions and the bulk of the imperial court, there were several potential rivals. Four respected proconsular lieutenants of Trajan were executed shortly after Hadrian's accession, forever marring his relationship with the Senate. Lucius Quietus a Mauretanian general who was largely in charge of suppressing the Jewish revolts that followed the Parthian invasions, Cornelius Palma, another Legate and former governor of Syria, Avidius Nigrinus, governor of Dacia and Publilius Celsus were killed for 'plotting against Hadrian'. Despite the fact that it was the Senate who ordered the executions, it was most likely done at the behest of one of Hadrian's agents. That agent perhaps was Attianus, still serving as praetorian prefect for his former ward. Hadrian, probably to avoid both political and public backlash, forever denied any involvement in the affair whatsoever and proclaimed his innocence publicly, even joining the sentiments of his predecessors Nerva and Trajan who swore to never harm a member of the Senate. However, the continued favor shown to Attianus and lack of any retaliatory response would seem to indicate that despite his 'innocence' he found the executions perfectly acceptable. Had Hadrian simply accepted responsibility, acknowledging personal enmity and jealousy over his succession as reasons for the executions, his strained relationship with the Senate may have been avoided.
Despite the perilous start to his reign, the new emperor, Imperator Caesar Traianus Hadrianus Augustus, would prove to be an effective yet completely different sort of leader. Not only would he immediately give up some of Trajan's territorial gains (another unpopular move with the above mentioned rivals) in order to bring the empire's borders more in line with the Augustan policy of using natural defenses, he would soon embark on an ambitious journey throughout the empire to inspect military dispositions, quell revolts and catalogue the affairs of Rome's provinces.