As the Empire expanded, two key frontiers revealed themselves. In the West, behind the rivers Rhine and Danube, Germanic tribes were an important enemy. Augustus, the first emperor, had tried to conquer them but had pulled back after the disastrous Battle of the Teutoburg Forest. Whilst the Germanic tribes were formidable foes, the Parthian Empire in the East presented the greatest threat to the Empire. The Parthians were too remote and powerful to be conquered and there was a constant Parthian threat of invasion. The Parthians repelled several Roman invasions, and even after successful wars of conquest, such as those implemented by Trajan or Septimius Severus, the conquered territories were forsaken in attempts to ensure a lasting peace with the Parthians. The Parthian Empire would be succeeded by the Sasanian Empire, which continued hostilities with the Roman Empire.