From the spring of 39 to the autumn of 38, Agrippa was proconsul of Gaul, where he laid the foundations of Roman rule north of the Alps. Until then, the province had been left to its own: it had been conquered by Julius Caesar, but the civil wars had prevented decent administration. This was now to change. First, Agrippa crossed the river Rhine and fought on behalf of the Ubians a war against the Suebians, a Germanic tribe that was notoriously aggressive. After the campaign, Agrippa resettled the Ubians on the west bank of the Rhine, and founded Ara Ubiorum, a city that became better known as Cologne. At the same time, a part of the tribe of the Chatti migrated to the Lower Rhine and Waal, where it mixed with the natives and accepted their ancient name: Batavians, "people of the fertile wetlands". Their capital was Batavodurum, modern Nijmegen.
Agrippa also built several roads: one from the Gallic capital Lugdunum (Lyons) to Aquitania in the west (where a rebellion was suppressed in 38), one to the Upper Rhine in the northeast, and one to Langres in the north, where it divided into two branches: to the northwest, to Reims and the Channel, and to the north, to Trier and Cologne. The presence of Italian wine amphora in towns along the new roads proves that merchants followed in the footsteps of the soldiers.