At the start of the war, Trajan built another bridge over the Danube to move his legions faster into Dacia. Unlike the first war, the second war involved several skirmishes that proved costly to the Roman military, who, facing large numbers of allied tribes, struggled to attain a decisive victory. An assault against the capital Sarmisegetusa took place at the beginning of the summer of 106 with the participation of the legions II Adiutrix and IV Flavia Felix and a detachment (vexillatio) from Legio VI Ferrata (see also Battle of Sarmisegetusa). The Dacians repelled the first attack, but the Romans destroyed the water pipes to the Dacian capital. The city was burned to the ground. Decebalus committed suicide rather than face capture by the Romans. Nevertheless, the war went on. Due to the treason of a confidant of the Dacian King, Bicilis, the Romans found Decebalus's treasure in the River Sargesia – a fortune estimated at 165,500 kg of gold and 331,000 kg of silver. The last battle with the army of the Dacian king took place at Porolissum.
Dacian territories annexed to the Roman Empire (marked in red)
In 113, Trajan built Trajan's Column near the Colosseum in Rome to commemorate his victory. Although the Romans conquered the ancient Kingdom of Dacia, a large remainder of the land remained outside of Roman Imperial authority. Additionally, the conquest changed the balance of power in the region and was the catalyst for a renewed alliance of Germanic and Celtic tribes and kingdoms against the Roman Empire. However, the material advantages of the Roman Imperial system wasn't lost on much of the surviving aristocracy. Thus, most of the Romanian historians and linguists believe that many of the Dacians became romanized (see also Origin of Romanians).