On coming into office, Tiberius was the most experienced general of his age. He had fought long wars in the West, along the German and Danubian frontiers. One might have expected him to continue the aggressive policies of Augustus, but he immediately announced that his intent was to keep the Empire safe within its current boundaries rather than expand.
He maintained that policy in general. The Romans engaged in campaigns in Germany seemingly simply to give the soldiers direction in life. Drusus was engaged in some form of campaign along the Danube in AD 18 (Tacitus, Annales 2.62) which appears to have put pressure on the German tribes. The revolt of the Frisii suggests continued problems in the region, but there was no major campaigning recorded for late in the reign.
Suetonius (Tiberius 41) produces a litany of military disasters due to Tiberius’ administrative mismanagement of Armenia, Moesia and Gaul. It is unclear to what events in Gaul and Moesia Suetonius’ précis was referring. Tacitus’ account of the problems in Armenia and Parthia suggests rather that Tiberius, through the effective diplomacy and military tactics of Lucius Vitellius, was initially successful in supporting the Roman’s preferred candidate for the Parthian throne (Tacitus, Annales 6. 31–7) until the puppet’s support dissolved (Tacitus, Annales 6. 41–4). In any case, Rome was to secure a peace treaty and brought stability to the borders, though this had to wait until the reign of Gaius (Dio, 59. 27.3–4; Suetonius, Gaius, 14.3). The policy continued the general successful but pacific interventions in the East of Augustus and which was also continued by Germanicus.
There were revolts. The Numidian leader Tacfarinas revolted in AD 17 (Tacitus, Annales 2.52). Although the Romans proclaimed victory, Tacfarinas remained in revolt through AD 20 when a series of inconclusive engagements were fought (Tacitus, Annales 3. 20-21). The war escalated and Tiberius appointed Junius Blaesus, Sejanus’ uncle, to the command (Annales 3.32; 3.35; 3.72-74). He was replaced in AD 24 and the war was eventually brought to an end by Publius Cornelius Dolabella (Annales 4.23-26).
A disturbance in Thrace was suppressed in AD 21 (the same year as the Florus and Sacrovir revolt) (Annales 3.38-39). The region continued to be troubled at least until AD 26 with a major campaign fought in the region (Annales 4.46–51)
As a whole, Tiberius followed a very conservative policy in the provinces. He seems to have dealt with those military problems that arose, but to have avoided any major expansion, especially in the latter years of his reign. His largely pacific policy left the empire undamaged and avoided the military disasters that had marred the final years of Augustus’ reign.
The policy may partly have been determined by his lack of reliable family members to act as generals after the death of Drusus. But even before then, Germanicus’ aggressive policies had been restrained and although Drusus spent much of his time in the northern Balkans, he was not engaged in expansionist wars.
Tiberius has won enough battles and fought enough wars. He did not need to accumulate any further credit through military actions. War was not a priority for the aged emperor.