Before and after the fall of the Western Roman Empire, most provinces and ex-provinces of the empire were engulfed in constant war as a consequence of the Germanic invasions that began in 409 A.D., which in combination with other factors brought about a number of negative effects on the size of the European population in comparison to its size during the best years of the Roman Empire:
Even before the Germanic invasions, the Roman government had raised the taxes so high, and bound the farmers to their lands, so that many farmers became indebted to great landowners, and the only way they had to pay the high interest rates on the loans they borrowed was indenture servitude (the debtor has to work for the creditor until the debt is paid off, something that never happened). In many cases, farmers fled indenture servitude and died because of starvation, armed robberies or as innocent victims of fights between armed Germanic bands. This situation depopulated formerly highly populated provinces like Gaul.The constant decrease in food production because of farmers fleeing from tax collectors and/or indenture servitude brought about a shortage of food in the cities and a decrease in the birth rate.The shortage of food also affected the nutrition of the population and made them prone to illness and disease and an increase in the mortality rate.Before the Germanic invasions , literacy in the Western Roman Empire was high and the amount of books owned per person was high as well. Because of the violence of the invasions, many books were destroyed as households were burned down or abandoned, and people because more concerned with survival than literacy. Only Church clergymen remained literate.Since nearly all books containing all the theoretical and practical knowledge on technological subjects were lost during the Germanic invasions, many of the advances achieved by Greek and Romans became unavailable until rediscovered by the Arabs and reintroduced to Europe in the Late Middle Ages.The breakdown of the Roman bureaucracy and cease of tax collection resulted in the neglect of cities, roads, aqueducts,city public buildings  maintenance and other important elements of infrastructure. As cities no longer received food from the countryside, their inhabitants emigrated to the countryside in search for food. By 408 AD, Rome had a population of nearly one million inhabitants. One century later, its population would not surpass 10,000 inhabitants.
