Late Antiquity – broadly defined here as the third to the early seventh century ad – is a period which offers a geopolitical scenario full of intriguing dynamics and possibilities in relation to diplomacy and foreign relations. In contrast to the early Roman Empire of the first two centuries ad, the late Roman Empire was no longer a great power without serious rivals. To the east there was now another major state of substance in the form of Persia, ruled by the centralising and ambitious Sasanian dynasty, while to the north the fragmented Germanic tribes of earlier centuries had coalesced into larger units capable of presenting the empire with more serious challenges. In these changed circumstances, warfare assumed renewed importance in protecting the empire's interests, but so too did diplomacy, a subject about which much relevant source material has survived from these centuries. There are a number of major historians whose narratives include substantial focus on war and peace, notably Ammianus Marcellinus for the third quarter of the fourth century and Procopius for the first half of the sixth. There are also a number of other historians whose work survives only in the form of excerpts. These are more valuable than one might otherwise suspect, because many of these excerpts are preserved as a result of their inclusion in handbooks produced by a later Byzantine emperor (Constantine VII Porphyrogenitus (913–59)) for the purpose of providing his son with guidance on the conduct of foreign affairs – so their content is helpfully skewed in favour of diplomacy.