Before Trajan returned to Rome in AD 99 to assume his place as 'emperor', time spent scouting enemy dispositions and investigating the Danube fortifications assuredly inspired him to prepare for an offensive into Dacia. Earlier campaigns against the Dacians as well as against Germanic tribes across the Danube by Domitian had met with some success, but the situation had been largely left unsettled. The Dacian King Decebalus, who had remained in power as a thorn in the proverbial Roman side, had spent the better part of the last decade securing his position and preparing armies in the Roman style. Thanks in large part to engineers provided by Domitian, Decebalus had fortified the approaches into his kingdom and created a formidable obstacle to Roman dominance of the region. As Decebalus raised his status to one that was among the most capable enemies in Roman history, Trajan had little choice but to plan a campaign to eliminate the threat.

Though the new emperor was a soldier at heart, he also understood the necessity of making political arrangements in Rome before entirely focusing his energy elsewhere. The year AD 100 was spent in Rome both honoring Nerva, ultimately with deification, and building a sense of governing authority within the Senate. The imperial court was minimal in comparison to previous 'administrations' and Trajan preferred a low key approach to government. Throughout his reign he would rely upon provincial governors to make decisions on their own merits and defer to the emperor only it matters of extreme importance. Thanks to Pliny the Younger, the provincial governor whose correspondence with Trajan is largely extant, a vivid portrait of Trajan's style can be seen (despite the terrible lack of information from other ancient historians). In effect, his rule was much like that of a general using subordinate officers in a military sense. Like Nerva, Trajan continued the popular measures that punished the delators (informers) for their part in creating administrative disorder and he reduced the power of the Praetorians and reformed the court system. For his deeds in continuing and perfecting the social welfare system known as the alimenta, he earned the name Optimus, or best, from the people. In the short time that he stayed in Rome, Trajan prepared the Roman world for 60 years of steady and effective leadership.

All the while he stayed in Rome however the emperor's sights were set across the Danube. The great historian Tacitus, a contemporary of Trajan, had published his work 'Germania' in this time period (one of the few ancient sources dedicated to the little known Germanics) initiating public propaganda that would support military expeditions to the north. Though Trajan's target never quite fell in line with Tacitus' expectations, there was likely wide public and aristocratic approval, thanks in part to the historians' efforts. Trajan raised two legions prior to launching an attack, II Traiana and XXX Ulpia (so numbered as it was the 30th active legion at the time) and shifted existing legionary placement to support a large scale campaign. By the spring of AD 101 plans were settled and the emperor marched north with an army that would eventually involve as many as 11 legions.

Unfortunately the details of Trajan's campaign are largely lost to history. Dio Cassius' account is partially fragmented and lacking the military attentions of past writers, such as Caesar. The other notable historians of the era, Suetonius, Tacitus, etc. did not focus much attention on the reigns of contemporary emperors but instead wrote largely about events prior to their own lifetimes thereby leaving Trajan ignored by the most common ancient resources. Despite this, we do know that the Dacian War was a combination of difficult fighting mixed with marvels of engineering. The defenses of Decebalus were impressive and Trajan was required to use the vaunted discipline and perseverance of the Roman army to succeed. With the service of the engineer Apollodorus of Damascus, the Romans completed immense road works along the Danube, begun a century earlier under Tiberius, and defeated the difficulty of logistics and hazardous terrain. In a great feat of engineering and architecture, Apollodorus designed a road straight up to and through the Iron Gates of the Danubian gorges by cantilevering it from the sheer rock face. This marvel of technology essentially allowed the legions to walk on top of the river as they made their way into the Carpathian Mountains.

A significant battle was fought at Tapae in AD 101 (the sight of a previous battle under Domitian in AD 89) on the approach to the Dacian capital of Sarmizegethusa. According to Dio Cassius keeping with the tradition of Trajan as a compassionate commander, "(Trajan) engaged the foe, and saw many wounded on his own side and killed many of the enemy. And when the bandages gave out, he is said not to have spared even his own clothing, but to have cut it up into strips. In honor of the soldiers who had died in the battle he ordered an altar to be erected and funeral rites to be performed annually." Soon after the Romans advanced upon the Dacian capital and Decebalus was forced to capitulate. Surprisingly, the powerful leader was left in power as a client king to Rome, but he agreed to raze his fortresses, surrender weapons and prisoners and likely paid a sizeable tribute (though Dio doesn't provide any detail.)

Temporarily victorious and having been rewarded the title Dacicus, Trajan returned to Rome to celebrate a triumph. He entertained the people with gladiatorial games and rewarded his officers for their service in the campaign. The celebration was short-lived though as Decebalus may have mistakenly compared the conviction of Trajan to that of Domitian. After his 'defeat' to Domitian (in which the Romans actually paid a tribute to Decebalus in order to keep the peace) he was allowed to rebuild his army and defense systems virtually unopposed. After this more recent defeat however, similar actions that effectively broke the peace arrangements were met with swift and decisive imperial response.

In preparation for another Danube crossing, Trajan again turned to his engineer Apollodarus for project of unprecedented military grandeur. A massive stone bridge spanning over 3,500 feet (1,100 meters) in length and 60 feet (19 meters) in width was built using enormous wooden arches set upon 20 stone and cement pillars. Built over the course of 2 years, this bridge became the primary source of traffic to and from Dacia. Despite finally being purposely destroyed a century and a half later by Aurelian as the Romans pulled out of Dacia, it was to remain for more than a thousand years the longest bridge that had ever been built. In AD 105 the new campaign was launched, and according to Dio, "Trajan, having crossed the Danube by means of the bridge, conducted the war with safe prudence rather than with haste, and eventually, after a hard struggle, vanquished the Dacians. In the course of the campaign he himself performed many deeds of good generalship and bravery, and his troops ran many risks and displayed great prowess on his behalf." A hard fought war lasting for just over a year ended with the Romans once again descending upon the Dacian capital of Sarmizegethusa. This time however, there was to be no respite for Decebalus. Rather than retain the right to rule as a client King, he was forced to flee. Rather than end up as a trophy in Trajan's second Dacian triumph, Decebalus was eventually forced to take his own life.

Dacia was immediately annexed as an imperial province, and many new colonies were founded laying a foundation for Roman influence that lasts into the present day. In fact, Romania, the modern equivalent to a portion of Dacian territory speaks a language that claims to be the most closely related modern tongue to that of ancient Latin. Additionally, again thanks to Apollodorus, Trajan left two impressive monuments commemorating his victory. The Tropaeum Traiani (restored in the late 20th century after 2 millennia of slow decay) in modern Adamclisi stands atop a hill and its visibility from great distances was a constant reminder of Rome's power. In Rome, Apollodorus built one of the great lasting treasures of imperial architecture. Trajan's Column is a sculpted pillar, standing 100 Roman feet tall, with 23 rings depicting a vast assortment of images relaying the history of the Dacian Wars. While it served as an unparalleled piece of propaganda glorifying the emperor and his achievements, it also stands today as an invaluable primary source of information into the Roman legions and ancient warfare.

When Trajan returned to Rome in AD 106, he did so with a vast treasure. According to Trajan's ancient contemporary Cryton (whose original work was lost but retold through Ioannes Lydus in the 6th Century) 'the sum of five million gold and twice as much silver' was taken. Though this number is difficult to translate into modern equivalents (and must be measured with a sense for ancient exaggerations) one scholar, Jerome Carcopino, roughly translated this immense sum to 180 tons (165,000 kg) of gold and 360 tons (331,000 kg) of silver. With this enormous haul Trajan was able to secure the health of the imperial treasury, finance projects such as the draining of the Pontine Marshes and provide a triumph that was unrivaled in the long history of Rome. 123 straight days of gladiatorial games were held, including fights between as many as 10,000 pairs of gladiators and the death of 11,000 animals. With his popularity among the people and the aristocracy at an unrivaled peak, even embassies from as far away as India came to Rome to pay respects to the man who was commonly becoming regarded as the greatest Emperor since Augustus, if not the greatest of all.