War and expansion

Extent of the Roman Empire under Augustus. The yellow legend represents the extent of the Republic in 31 BC, the shades of green represent gradually conquered territories under the reign of Augustus, and pink areas on the map represent client states; however, areas under Roman control shown here were subject to change even during Augustus' reign, especially in Germania.

Imperator Caesar Divi Filius Augustus chose Imperator, "victorious commander" to be his first name, since he wanted to make the notion of victory associated with him emphatically clear. By the year 13, Augustus boasted 21 occasions where his troops proclaimed "imperator" as his title after a successful battle. Almost the entire fourth chapter in his publicly released memoirs of achievements known as the Res Gestae was devoted to his military victories and honors. Augustus also promoted the ideal of a superior Roman civilization with a task of ruling the world (the extent to which the Romans knew it), a sentiment embodied in words that the contemporary poet Virgil attributes to a legendary ancestor of Augustus: tu regere imperio populos, Romane, memento—"Roman, remember by your strength to rule the Earth's peoples!" The impulse for expansionism, apparently prominent among all classes at Rome, is accorded divine sanction by Virgil's Jupiter, who in Book 1 of the Aeneid promises Rome imperium sine fine, "sovereignty without limit".

Bust of Tiberius, a successful military commander under Augustus before he was designated as his heir and successor

By the end of his reign, the armies of Augustus had conquered northern Hispania (modern Spain and Portugal), the Alpine regions of Raetia and Noricum (modern Switzerland, Bavaria, Austria, Slovenia), Illyricum and Pannonia (modern Albania, Croatia, Hungary, Serbia, etc.), and extended the borders of the Africa Province to the east and south. After the reign of the client king Herod the Great (73–4 BC), Judea was added to the province of Syria when Augustus deposed his successor Herod Archelaus. Like Egypt which had been conquered after the defeat of Antony in 30 BC, Syria was governed not by a proconsul or legate of Augustus, but a high prefect of the equestrian class. Again, no military effort was needed in 25 BC when Galatia (modern Turkey) was converted to a Roman province shortly after Amyntas of Galatia was killed by an avenging widow of a slain prince from Homonada. When the rebellious tribes of Cantabria in modern-day Spain were finally quelled in 19 BC, the territory fell under the provinces of Hispania and Lusitania. This region proved to be a major asset in funding Augustus' future military campaigns, as it was rich in mineral deposits that could be fostered in Roman mining projects, especially the very rich gold deposits at Las Medulas for example.
Conquering the peoples of the Alps in 16 BC was another important victory for Rome since it provided a large territorial buffer between the Roman citizens of Italy and Rome's enemies in Germania to the north. The poet Horace dedicated an ode to the victory, while the monument Trophy of Augustus near Monaco was built to honor the occasion. The capture of the Alpine region also served the next offensive in 12 BC, when Tiberius began the offensive against the Pannonian tribes of Illyricum and his brother Nero Claudius Drusus against the Germanic tribes of the eastern Rhineland. Both campaigns were successful, as Drusus' forces reached the Elbe River by 9 BC, yet he died shortly after by falling off his horse. It was recorded that the pious Tiberius walked in front of his brother's body all the way back to Rome.

Gold mine of Las Médulas.

Muziris in the Pandyan Kingdom of Southern India, as shown in the Tabula Peutingeriana, with depiction of a "Temple of Augustus" ("Templum Augusti"), an illustration of Indo-Roman relations in the period.

To protect Rome's eastern territories from the Parthian Empire, Augustus relied on the client states of the east to act as territorial buffers and areas which could raise their own troops for defense. To ensure security of the Empire's eastern flank, Augustus stationed a Roman army in Syria, while his skilled stepson Tiberius negotiated with the Parthians as Rome's diplomat to the East. Tiberius was responsible for restoring Tigranes V to the throne of the Kingdom of Armenia. Yet arguably his greatest diplomatic achievement was negotiating with Phraates IV of Parthia (37–2 BC) in 20 BC for the return of the battle standards lost by Crassus in the Battle of Carrhae, a symbolic victory and great boost of morale for Rome. Werner Eck claims that this was a great disappointment for Romans seeking to avenge Crassus' defeat by military means. However, Maria Brosius explains that Augustus used the return of the standards as propaganda symbolizing the submission of Parthia to Rome. The event was celebrated in art such as the breastplate design on the statue Augustus of Prima Porta and in monuments such as the Temple of Mars Ultor ('Mars the Avenger') built to house the standards.
Although Parthia always posed a threat to Rome in the east, the real battlefront was along the Rhine and Danube rivers. Before the final fight with Antony, Octavian's campaigns against the tribes in Dalmatia was the first step in expanding Roman dominions to the Danube. Victory in battle was not always a permanent success, as newly conquered territories were constantly retaken by Rome's enemies in Germania. A prime example of Roman loss in battle was the Battle of Teutoburg Forest in AD 9, where three entire legions led by Publius Quinctilius Varus were destroyed with few survivors by Arminius, leader of the Cherusci, an apparent Roman ally. Augustus retaliated by dispatching Tiberius and Drusus to the Rhineland to pacify it, which had some success although the battle of AD 9 brought the end to Roman expansion into Germany. The Roman general Germanicus took advantage of a Cherusci civil war between Arminius and Segestes; they defeated Arminius, who fled that battle but was killed later in 21 due to treachery.