The Pre-Roman Iron Age, spanning from the 4th to the 1st century BCE, marked a period of significant change for Denmark and southern Scandinavia. The climate grew cooler and wetter, making agriculture more challenging and prompting some local groups to migrate southward into Germania. During this time, the inhabitants of Denmark began to extract iron from peat bogs, which represented a crucial technological advancement. Evidence of strong Celtic cultural influences emerged, especially visible in the oldest Danish place names, indicating that Denmark was part of a broader cultural and trade network across northwest Europe.
Although the Roman Empire's borders did not extend into Denmark, trade routes connected the Danish people with the Roman provinces, as demonstrated by the discovery of Roman coins in the region. The presence of Roman artifacts, particularly from the 1st century CE, suggests that there were active exchanges between Denmark and the Roman world, and some Danish warriors may have even served in the Roman army. The earliest runic inscriptions in Denmark, dating back to around 200 CE, also indicate a blending of influences and a growing sense of a unique proto-Danish identity. The depletion of cultivated land towards the end of the Pre-Roman Iron Age contributed to increased migrations and conflict in northern Europe, particularly with Teutonic tribes moving into Roman territories in Gaul.
During the Nordic Iron Age, the Danes were primarily based in present-day Zealand and Scania, as well as neighboring parts of modern Sweden. At this time, Jutland was inhabited by two other Germanic tribes: the Jutes in North Jutland and the Angles in South Jutland, particularly in the region known as Angeln.
By the 5th century CE, Denmark entered the Germanic Iron Age, a period characterized by widespread migrations across Northern Europe. This era set the stage for the emergence of distinct Germanic tribes, laying the foundations for the later development of Danish society and the broader Scandinavian cultural identity Around the middle of the first millennium CE, both Jutland and Angeln were incorporated into the emerging Danish kingdom or kingdoms. This expansion also included southern Schleswig (now part of northern Germany), where the Danes built the Danevirke, a large set of fortifications that served to mark the southern boundary of their realm. The Danevirke was extended several times in the following centuries.
During this period, religious practices often included ritual sacrifices, both animal and human, with bodies being immersed in peat bogs. This ritual practice has left behind incredibly well-preserved bog bodies, such as the Tollund Man and the Grauballe Man, which offer remarkable insights into the life, customs, and beliefs of the Danish people during the Iron Age. These findings reveal a society deeply connected to its environment and engaged in complex religious traditions.