What is important to understand when answering this question is that the rise of central Europe, both in population and in prominence, is closely linked to the decline of the Roman Empire.

While the decline of Rome is hotly contested and deserving of its own library of history, the Crisis of the Third Century will be the most important “start date” in my response to this question.

Crisis of the Third Century

The Crisis of the Third Century was a period of historic civil unrest and destruction for the Roman Empire. With the succession of the Gallic and Palmyrene empire in addition to a series of short dynasties, the 3rd century as whole is often regarded as the beginning of the end of the Roman Empire. While Aurelian (270-275) was able to reunite the Empire and Diocletian (284) was able to solidify it, fundamentally much of the power in the West was irreparably weakened. This meant that many of the previous defenses used on the Western European borders of the Empire were also permanently damaged. The weakening of the Empire also led to a breakdown of non-traditional defense mechanisms for the Empire. Often, instead of exerting sheer military force on its borders, Rome relied on alliances with local chieftains and tribesmen. Along the border, tribal groups were propped up with imperial support to act as a buffer between formal Roman territory and that which it had not yet conquered. Along with the weakened military defense resulting from the civil wars of the 3rd century, the economic losses and breakdown of internal trade meant that alliances at the border also began to fray.

This long introduction to the rise of central Europe is to illustrate how connected the fate of Northern and Central Europe was to the rise and fall of the Roman Empire, and how its decline created a period of mass transition in the continent.

Migration Period

Following the crisis of the 3rd century, Germanic tribes began to gain more power in the region. The Migration Period, roughly defined as the period of 375 to 568, heralded the increased mobility and power of Germanic tribes in Europe. Spurred by the Hunnic invasions of Europe and a decrease in centralized authority, the Migration Period signaled a change in the power structure and demographics of Western Europe. The transition from Roman province to “barbarian” rule was inconsistent across Western Europe, Roman collapse in Britannica did not follow the same path as in Gaul or in Hispania, etc. Fundamentally, however, the outcome was the same, with the collapse of a centralized military force and power in Europe being replaced by many smaller groups. The sack of Rome by the Visigoths in 410 under the leadership of Alaric is often cited as a key point in this period, symbolizing the “defeat” of the Roman Empire under the power of the “barbarians”.

Merovingians and Carolingians

Out of the changing demographics and power structures of the Migration Period, there arose several smaller but notable kingdoms in central Europe. While some retained their connection to Rome, many decided to forge an identity outside of it. The Merovingian Dynasty, formally founded by Childeric I (481) but expanded and formalized by his son Clovis I (ascended 468), is an example of one of the many kingdoms that arose during the Migration Period. The Merovingians occupied parts of Western Europe including much of France and Germany (the area of interest initially written in your question). The kingdom was divided among Clovis I’s sons upon his death and later reunited, continually in a process of division and reunification. While the Merovingians are referred to under a common name, like the rest of Europe they were not a homogenous population but rather a combination of Germanic, Frankish and formerly Roman populations.

The Merovingian Dynasty eventually gave way to probably the most famous “dark age” dynasty; the Carolingians. The overthrow of the Merovingian line and the crowning of Pepin the Short in 751 was the formal beginning of the Carolingian dynasty. Arising from a series of civil wars and infighting as well as papal consent for overthrow, the Carolingian line would give birth to one of the most famous medieval European rulers (Charlemagne) as well as provide the groundwork for the Holy Roman Empire. Charlemagne, son of Pepin the Short, significantly expanded the ruling territory of the Carolingians, notably adding on Bavaria, Saxony and the Lombard Kingdom, as well as uniting several of the petty kingdoms across the Carolingian territory. Charlemagne’s reputation as “the Father of Europe” does not come without reason. The Carolingian dynasty began to crumble following his death, with Carolingians practicing the same territorial division among sons/grandsons as their Merovingian predecessors. The Carolingian line formally ended in 1122. The Carolingians left many lasting impacts on Western Europe, one of the most notable being the Carolingian Renaissance, which saw an expansion in art and architecture.

The history of central Europe from the decline of Rome is complicated and fascinating and worth a deeper exploration than what could be summed up in this answer let alone in a library of books.. However, fundamentally, it was connected to a decline of Rome and a relative “power vacuum” that was left as a result of decreased centralized power. The Migration Period allowed for a great deal of demographic and cultural change which was compounded with a receding Roman Europe. Larger kingdoms such as the Merovingians and later Carolingians led eventually to a new power structure centered in central Europe, never encompassing nearly the territory that the Roman Empire did at its height.

TL;DR: The collapse of Rome had ripple effects across Europe. With no central power, the Migration Period ensued, leading to rise in power of kingdoms across central Europe, which populated the once more scantly populated Germania.

Sources Consulted (highly highly recommend Wickham’s book)

Collins, Roger (1999). Early Medieval Europe, 300–1000. New York: Palgrave Macmillan.

Heather, Peter (2013). The Restoration of Rome: Barbarian Popes and Imperial Pretenders. Oxford and New York: Oxford University Press.

Wickham, C. (2010). The inheritance of Rome: A history of Europe from 400 to 1000. London: Penguin Books.

Gibbon, Edward (1839). Hilman, H. H. (ed.). The History of the Decline and Fall of the Roman Empire.

Halsall, Guy. Barbarian Migrations and the Roman West, 376–568. Cambridge University Press, 2007.

Mayr-Harting, Henry. “Charlemagne, the Saxons, and the Imperial Coronation of 800.” The English Historical Review, vol. 111, no. 444, 1996, pp. 1113–1133. JSTOR, www.jstor.org/stable/575852. Accessed 29 May 2021.

“The Long Third Century.” Imperial Rome AD 284 to 363: The New Empire, by Jill Harries, Edinburgh University Press, Edinburgh, 2012, pp. 1–24. JSTOR, www.jstor.org/stable/10.3366/j.ctt1g0b463.8. Accessed 29 May 2021.