There were two major volcanic eruptions in 536 and 542 AD which led to a period known as the Late Antiquity Little Ice Age. It is suspected poorer diets from poorer harvests led to the Plague of Justinian wrecking havoc with a weakened population.

While there was a decline the Avars, Sassanid and Gupta civilisations, the vacuum led to the rise of Turks, Mongols and Islam. The strategy they have in common is that, at the time, the peoples lived nomadic lifestyles on the fringes of deserts. With the Sun being ‘feeble’, as Roman senator Cassiodrus puts it, these people could be more productive during the day.