page_title: Imperial Roman army
section_title: Imperial expansion strategy
hierarchical_section_title: Imperial Roman army / Historical development / Imperial expansion strategy
caption_attribution_description: Emperor Marcus Aurelius (161-180 AD) shows his clemence towards the vanquished after his success against Germanic tribes. Bas-relief from the Arch of Marcus Aurelius, Rome, now in the Capitoline Museum in Rome.
context_page_description: The Imperial Roman army was the terrestrial armed forces deployed by the Roman Empire from about 30 BC to 476 AD, the final period in the long history of the Roman army. This period is sometimes split into the Principate and Dominate periods.
  Under Augustus, the army consisted of legions, eventually auxilia and also numeri.
  Legions were formations numbering about 5,000 heavy infantry recruited from the ranks of Roman citizens only, transformed from earlier mixed conscript and volunteer soldiers serving an average of 10 years, to all-volunteer units of long-term professionals serving a standard 25-year term.
  Auxilia were organised into regiments of about 500 strong under Augustus, a tenth the size of legions, recruited from the peregrini or non-citizen inhabitants of the empire who constituted approximately 90 percent of the Empire's population in the 1st century AD. The auxilia provided virtually all the army's cavalry, light infantry, archers and other specialists, in addition to heavy infantry equipped in a similar manner to legionaries.
  Numeri were allied native units from outside the Empire who fought alongside the regular forces on a mercenary basis.
context_section_description: Under Augustus, the European borders of the empire he inherited from his grand-uncle Julius Caesar were considerably expanded. During the first half of his sole rule (30–9 BC), Augustus' central strategic objective was to advance the Roman border from Illyricum and Macedonia to the line of the  Danube, Europe's greatest river, in order both to increase strategic depth between the border and Italy and to provide a major fluvial supply route for the Roman armies in the region. The strategy was successfully executed: Moesia (29-7 BC), Noricum (16 BC), Raetia (15 BC) and Pannonia (12-9 BC) were annexed in steady succession. After settling the Danube border, Augustus turned his attention to the North, where Julius Caesar had in 51 BC established the border of Roman Gaul along the river Rhine, the second major European fluvial route. Augustus launched an ambitious strategy of advancing the Rhine border to the river Elbe, aiming to incorporate all the warlike Germanic tribes. This would eliminate their chronic threat to Gaul, increase strategic depth between free Germans and Gaul, and make the western Germans' formidable manpower available to the Roman army. But a massive and sustained military effort (6 BC – AD 9) came to nothing. Roman advances in Germania Magna (i.e. Germany outside the empire) had to be scaled down during the Great Illyrian Revolt of AD 6–9, when many troops were diverted to Illyricum. Then Augustus' expansion strategy suffered a crushing setback when some 20,000 Roman troops were ambushed and massacred by the Germans at the Battle of the Teutoburg Forest in AD 9. After this, Augustus shelved his Elbe strategy. It was apparently revived briefly by his successor Tiberius, whose nephews, the generals Germanicus and Drusus, launched major and successful operations in Germania in AD 14–17, during which the main tribes responsible for Varus' defeat were crushed and the three lost legionary aquilae (eagle-standards) were recovered.
  But if Tiberius ever contemplated advancing the border to the Elbe, by AD 16 he had clearly abandoned the idea and decided to keep the border at the Rhine. Most likely, he assessed the Germanic tribes as too powerful and rebellious to incorporate successfully into the empire. After this, plans to annex western Germania were never seriously revived by Augustus' successors. Under the Flavian emperors (69-96), the Romans annexed the trans-Rhenane region they called the Agri Decumates i.e. roughly the territory of the modern southwestern German state of Baden-Württemberg. But this acquisition was strictly aimed at shortening the lines of communication between the legionary bases of Germania Superior and Raetia provinces (Mainz and Strasbourg in Germania Sup. and Augst and Regensburg in Raetia), by incorporating the salient between the upper reaches of the Rhine and Danube rivers. It was not part of a renewed effort to subdue Germany as far as the Elbe.
  Doubtless mindful of the costly failure of his Elbe strategy, Augustus reportedly included a clause in his will advising his successors not to attempt to expand the empire further. In the main, this advice was followed, and few major permanent annexations were made for the duration of the Principate. The main exceptions were (a) Britain, which was invaded by the emperor Claudius in AD 43 and was progressively subdued (as far as the Tyne-Solway, line of the later Hadrian's Wall) in 43–78. However, the stiff, prolonged resistance offered by native tribes seemingly confirmed Augustus' warning, and reportedly led the emperor Nero at one stage to seriously consider withdrawing from Britain altogether; and (b) Dacia, conquered by Trajan in 101–6. In both cases, it appears that, apart from the emperors' self-glorification, the primary motivations were probably the target-countries' mineral resources and also to prevent those countries becoming bases for anti-Roman resistance in Gaul and Moesia respectively.
  Apart from Britain and Dacia, other major territorial acquisitions by ambitious emperors were swiftly abandoned by their immediate successors, 
caption: Emperor Marcus Aurelius (161-180 AD) shows his clemency towards the vanquished after his success against Germanic tribes; bas relief from the Arch of Marcus Aurelius, Rome, now in the Capitoline Museum in Rome.
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