Seriously? Edward Gibbon is so last millennium. There's massive differences between the Germania of the Five Good Emperors and the Germania of Theodosian times. Up until the fourth century there were few modest settlements at all; most of the region was sparsely settled and there was little economic or demographic activity going on. By the time the Goths were on the Danube, though, continual contact with the Romans and reaction to Roman frontier politics had meant that trade had boomed, populations exploded, and Germanic kingdoms were beginning to form. If Trajan had to face the Germania of 400 AD instead of the Germania of 100 AD, he would've lost just as badly.Anyway, when Rome converted to Christianity, it's fate was sealed. The old Stoics and pantheism were part the enablers for conquest on philosophical grounds, whereas Christianity allows you to concede defeat without recompense. It would be as if Hitler didn't have his misinterpretations of Nietzsche telling him to constantly play the offensive for fear that losing the momentum would lose him the war.