Tacitus's description of Britain is not as interesting as his description of the Britons who lived there. His geography is awry for he believed that Britain was closer to Spain than it is, and that it faced the Atlantic only on the north as opposed to the north and west. Those errors though once noted are not particularly interesting- or are interesting only to the historians of the evolution of geography. Rather than look at his understanding of Britain, a cold country with oysters, I want to look at his understanding of the Britons- for in his introduction to them he brings out two themes which both mean that they have more virtue than the Romans that they opposed but also that they were doomed to be conquered and their lands to be distributed by those Romans. Tacitus here proposes a model for why, despite the fact that the barbarians had more military virtue than the imperial soldiers, the luxurious empire of Rome managed to conquer them.
Tacitus here expands his analysis of the effects of tyranny upon Roman society to an analysis of the effects of empire upon provincial society. He compares the Britons and the Gauls, the former
show more spirit: they have not yet been enervated by protracted peace. History tells us that the Gauls too had their hour of military glory: but since that time a life of ease has made them unwarlike: their valour perished with their freedom. The same has happened to those Britons who were conquered early (Agricola trans Mattingley para 11)
The point is important- imperial power dulls the martial spirit of the provinces. The Britons have been brought to a point on the scale which leads to slavery- they are now 'broken in to obedience but not as yet to slavery' (Agricola trans Mattingley 1970 para 13)- Tacitus's point is that Rome deserves to rule its servile states- its colonies because they have abandoned the mindset of liberty that they once had. Like the ROmans themselves who after Nero were unable to restore the Republic, in his view the provincials are now unable to govern themselves.
The logic is brutal and imperial- but it is also important for Tacitus's analysis linking together national freedom and national mindsets is important in later history. As an idea it provides the explanatory framework to Tacitus's history: firstly by answering the question of how a weakened Rome could keep the empire and secondly by demonstrating why Tacitus is a historian of the imperial centre and its armies and not of the provinces. Simply put the answer to the two questions is the same- the history of Europe turned thanks to Roman imperialism in Tacitus's view from the history of isolated republics and aristocracies into the history of a court in Rome with an enfeebled and servile caste of slaves (provincial and Roman, senator and slave) outside it.