Detailed information about the Germanic peoples is provided by the Roman general Julius Caesar, who campaigned in Germania in the 1st century BC. By this time Germanic peoples are believed to have dominated an area stretching from the Rhine in the west to the Vistula in the east, and the Danube in the south to Scandinavia in the north. Under Caesar's successor Augustus, the Romans sought to conquer the Germanic peoples and colonize Germania, but these efforts were significantly hampered by the victory of Arminius at the Battle of the Teutoburg Forest in 9 AD, which is considered a defining moment in German history. The early Germanic peoples are famously described in "Germania" by the 1st century Roman historian Tacitus. At this time, the Germanic peoples were fragmented into a large number of tribes who were frequently in conflict with both the Roman Empire and one another. By the 3rd century, Germanic peoples were beginning to merge into great coalitions, and had begun conquering and settling areas within the Roman Empire. During the 4th and 5th centuries, in what is known as the Migration Period, Germanic peoples overran the decaying Roman Empire and established new kingdoms within it. Meanwhile, formerly Germanic areas in parts of Eastern Europe were settled by Slavs.