It was Augustus who first realized that the Roman Empire could not go on expanding for ever. Horace could write
Caelo tonantem credidimus Iovem
Regnare; praesens Divus habebiiur
Augustus adiectis Britannis
Imperio gravibusque Persis;
but a very real part of Augustus' claim to grateful veneration lay in the fact that he made up his mind to leave Britons and Parthians alone—to seek in them not new subjects, but peaceful and respectful neighbours. Coercere intra terminos imperium was the advice he left to his successors; and in principle they never departed from it. Claudius might conquer Britain, Trajan Mesopotamia and Dacia; but these were “rectifications,” as we say nowadays, not obliterations, of the imperial frontier.
For the frontier of the Empire, as Augustus left it, was far from perfect. Tiberius, concerned above all to maintain intact the system created by Augustus, played here, as everywhere, a waiting game, and did not meddle with the Augustan frontiers. But his successor Gaius, or “Caligula,” may have contemplated a conquest, or at least an invasion, of Britain; he certainly made a demonstration on the shore of the Channel. And Claudius, the fourth Emperor, took the decisive step. Britain and Gaul were too close together, too intimately linked by geography, blood and civilization, to permit of an unfortified Channel frontier. Southern Britain was already in part Romanized, and the flag followed trade.