Yourcenar presumably means jade, which would have come from China or south-east Asia, and uses this as a spur for Hadrian to reflect on what might lie beyond the limits of the known world. In fact, no imported objects of jade have ever been found in a Roman context, but indirect trade did take place: Roman glass has been found on Chinese sites and Pliny the Elder, among other Roman writers, refers to the popularity of silk as a fashion accessory in Rome. The Chinese historical text, Hou Hanshu, refers to a Roman diplomatic delegation arriving at the court of Emperor Huan in 166 AD, sent by a ruler named "Andun" (安敦 in Chinese) usually thought to refer either to Antoninus Pius, Hadrian's immediate successor, or to the recipient of Yourcenar's fictional letter, Marcus Aurelius).
The passage that follows documents Hadrian's role (he was Legate of the V Macedonica Legion) in Trajan's military campaigns (101-106 AD) in Dacia (modern Romania). Although nominally a Roman ally, the local king, Decebalus, sanctioned numerous raids on Roman colonies across the Danube, and Trajan determined to suppress these. The campaign, graphically illustrated by the reliefs on Trajan's Column, ended with the defeat and suicide of Decebalus.
Lucius Julius Ursus Servianus (45-156 AD) was married to Hadrian's elder sister, Aelia Domitia Paulina. According to the Augustan History he did attempt to prevent Hadrian's travel to Germany following Nerva's death, but there is no evidence that he made an attempt on Hadrian's life at this point in time. The two men were subsequently reconciled, and Hadrian seems to have considered Servianus as a potential successor. By the time of Hadrian's death, however, Servianus was too old to succeed him and his (Servianus's) grandson, Gnaeus Pedanius Fuscus Salinator was considered a more likely option. Servianus was furious when Hadrian overlooked Salinator in favour of Antoninus Pius, and was subsequently implicated in a coup attempt against Hadrian, who ordered his death. According to Cassius Dio, Servianus exclaimed before he took his own life "my only prayer is that Hadrian lingers for a long time, praying for death but unable to die."
The cult of Mithra or Mithras, a god originally of Persian origin was one of a number of eastern cults (Christianity being another, as was the worship of Isis) which spread extensively across the Roman Empire in the late 1st and early 2nd Centuries AD. Mithraism in particular, which seems to have been exclusively male, gained a significant following in the Roman army, and its distinctive underground temples, built to resemble a cave, and always with a central image of Mithras slaying a bull, are found in many garrison towns. The cult is mentioned by Plutarch, Cassius Dio and others but they can tell us little about it since they were not themselves initiates. The 3rd/4th Century author, Porphyry tells us rather more, and this is probably Yourcenar's main source here, but Porphyry was writing at a time at which Mithraism was in decline, so his account may not reflect the beliefs and practices of Hadrian's time.
Heraclitus (c535-475 BC) was a pre-Socratic Greek philosopher who believed in the unity of opposites, and used the strung bow as a metaphor for this theory, an object whose form and function depends on an equilibrium between the tension of the string and the natural tendency of the wood. Stoic philosophers, including Marcus Aurelius, acknowledged his influence on their own work, and Yourcenar here explores the possibility that Hadrian, who was known to be a Hellenophile, may have influenced the younger man with his meditation on victory and defeat.
Vibia Sabina (83-136/137 AD) was the daughter of Salonina Matidia (Trajan's neice) and Lucius Vibius Sabinus. Her marriage to Hadrian was arranged by Trajan's wife, Plotina. It was by no means a love match: the marriage was childless, and Sabina is said to have had an affair with the historian, Suetonius.
The reference here is to Mark Antony's famous liaison with Cleopatra, and Titus's affair (before he became Emperor) with the Jewish princess, Berenike, which his father, Vespasian, forced him to end, doubtless alarmed by the precedent.
Here Yourcenar's Hadrian describes and accounts for the genesis of the historical Hadrian's policy of consolidation rather then further conquest, a policy which was continued under Antoninus Pius and Marcus Aurelius, and which found material expression in the construction of Hadrian's Wall.