`Cold, poverty and a life of danger and fatigue," opines Edward Gibbon, "fortify the strength and courage of barbarians.
"In every age they have oppressed polite and peaceful nations who neglected, and still neglect, to counterbalance these natural powers by the resources of military art."
Be that as it may, Gibbon quickly returns to his theme of The Decline and Fall of the Roman Empire: "And thus, the splendid days of Augustus and Trajan were eclipsed by a cloud of ignorance, and the barbarians subverted the laws and palaces of Rome."
Meteorologists, however, feel that the weather may well have been a part of the equation. They note that the northward spread of Roman culture coincided with an amelioration of the climate which followed a spell of cold conditions which had ended about 500BC.
Indeed, they can be even more precise: they point out that throughout its history the northern limits of the Roman Empire coincide closely with the 2.8 Celsius isotherm for January. In other words, the Romans never ventured anywhere if the average temperature in January was less than 2.8.
Gibbon more or less agrees, although he does not put a number on it. As to why the Romans stopped at Hadrian's Wall - and perhaps why they never tried to cross the Irish Sea - he says: "The masters of the fairest and most wealthy climates of the globe turned with contempt from the gloomy hills assailed by winter tempests, from lakes concealed in a blue mist, and from cold and lonely heaths over which the forest deer were chased by naked savages."
By the end of the fourth century, however, Europe's climate had begun to deteriorate again and a very severe winter in AD406 gave an unexpected advantage to the Romans' enemies.
That year, for the first time for centuries, the Rhine froze; the Romans still controlled the bridges and crossing points along the river, but ice gave the advancing Vandals unimpeded passage westwards.
Meanwhile in Italy, the copious rainfall which had graced the region was diminished greatly.
There were poor harvests and periodic famines and, because the previously swiftly flowing rivers of Italy could no longer be maintained, stagnant pools and marshes developed in the river beds and provided ideal conditions for frequent epidemics of malaria and bubonic plague.
As Gibbon puts it: "The winds might have diffused that subtle venom, but such was the universal corruption of the air that for many years the pestilence was not checked or alleviated by any difference of the seasons."
In such conditions did the hardy Vandals make their way inexorably towards Rome.