After the recall of Agricola from Britain, accusations of jealousy plagued Domitian. Coupled with renewed fears of a tyrannical rule (his appointment as perpetual censor granting him complete dominion over the assemblies in AD 85 as an example), Domitian still needed a major military victory and/or a distraction.
The Danube provided opportunities for both. Since the civil war of AD 69 that followed Nero's suicide and ended with the accession of Domitian's family the various Danube area tribes, Sarmatians, Marcomanni, Quadi, Dacians, etc., began to take advantage of Rome's pre-occupation. Raids were commonplace into frontier provinces such as Moesia and Roman efforts to stop it had little effect. As early as AD 70, the governor of Moesia, Fonteius Agrippa, was killed fighting these incursions and Vespasian, only recently gaining imperial power, did little but to strengthen various fortifications.
By the late 80's AD, the situation had become considerably more dangerous. The Dacians, a collection of small states and various tribes since the death of King Burebista in 44 BC, would be re-unified under what would become one of Rome's great adversaries, Decebalus (Diurpaneus). Translated from 'Strong as Ten Men' in Latin, the name itself seems to have been an honorary title among the Dacians and was much earned as he proved to be a formidable opponent to Rome for over 20 years.
In AD 85, the Dacians led a raid into Roman Moesia, again defeating and killing the governor Oppius Sabinus along with 2 legions. Domitian, along with his Praetorian Prefect Cornelius Fuscus, gathered their forces and traveled to the region to take command in person. The Dacians, despite proposals for peace, were pushed back across the Danube with some difficulty, but without any serious debilitating engagements. Domitian, likely viewing himself as the hero of the moment left the situation in the hand of Fuscus, while he returned to Rome to celebrate a terribly premature triumph in AD 86.
Later that same year Fuscus took matters into his own hands carrying out a planned punitive campaign across the river. This expedition, made up of as many as 6 legions was set the stage for the emergence of Decebalus. He met Fuscus at Tapae, within the extremely narrow mountain passed called 'Transylvania's Iron Gates' ambushed and repulsed the advance sending the Romans retreating back across the Danube. An entire legion was possibly obliterated as at least one standard was reportedly captured. Fuscus too paid the ultimate price giving his life in the fighting while Decebalus, by virtue of his great victory, went on to be named King of all the Dacians.
Domitian, understanding the severity of the situation returned to take direct command, but did so only within the safety of the Roman provinces. Moesia was reorganized into two separate provinces allowing for the presence of more legions under separate commands and two generals, Cornelius Nigrinus and L. Funisulanus Vettonianus arrived to conduct further military operations. The situation was stabilized and more legions were brought into the region, a major factor in the withdrawal from gains in Caledonia (Scotland) under Agricola, but the matter of the Dacians formidable presence on the border was yet to be settled.
Under Tettius Julianus the Romans regained some level of military dignity. In AD 88 he advanced along the same path as Fuscus and again met the Dacians at Tapae. This time the Romans were successful and drove the Dacians to retreat. However, the advance would be short-lived. Rumblings of revolt among the Rhine legions along with new attacks from Germanic tribes in Pannonia would prevent an all out Roman victory. While plans for further campaigns would continue to be made, Domitian was forced to accept an embarrassing treaty, paying off Decebalus and sending him skilled artisans to help with various infrastructure projects. Though the Romans did annex some land as a condition of the treaty, it was quite clear that Decebalus still held considerable sway, while Domitian faced continuing challenges in other areas.