Post-Roman and early medieval states are limited and defined by a variety of criteria, including topography and natural borders, population distribution, infrastructure, legislature and military capability. While the "great man" school of history is highly problematic, there's also an argument to be made for the influence of individual rulers on the waxing and waning of a nation state.

I tend to talk about the Anglo-Saxons a lot, but it's easy to draw two useful comparisons between Merovingian Francia and pre-conversion Anglo-Saxon England in c597, and England and Carolingian Francia and Ottonian Germany in the tenth and early eleventh century. With the effective total collapse of the Roman Imperial system in Britain, which had begun before the Anglo-Saxons even arrived, society had essentially fallen back upon pre-Roman 'tribal' structures. Without a logistical pool, established bureaucracy, codified legal system or well-maintained infrastructure, areas of control recede to those which can be easily controlled by the physical presence of a warlord or petty king. In Bede's Historia Ecclesiastica he talks of the Bretwaldas, kings of all England, but this is a position of largely symbolic overlordship and a system of tribute maintained through demands with menaces more than it is ever a form of direct rule.

While Merovingian Francia is certainly a troubled and at times precarious state, the continuation of the Church and the maintenance of at least some aspects of Roman Imperial logistics, bureauracy and infrastructure allow a government to maintain control over a much wider area. While an early Anglo-Saxon king may be able to reward personal loyalty with rings and trinkets in his hall, his Francian contemporary is able to reward distant service with grants of land. While it is a slow process, delayed by its contemporary progress in rival kingdoms, the conversion of England and the arrival of the Church allow for the coalesence of power and control to ever fewer nuclii, with a heptarchy of states emerging from the dozens of early kingdoms by the eighth century, reducing to five by the ninth century and 1 (occasionally three) by the 10th.

The longevity of a state's borders is influenced heavily by the means through which control is established, and the extent to which centralisation is maintained. During Charlemagne's reign, the loyalty of a far-flung nobility was maintained through regular aggressive military expansion, which bolstered personal loyalty to the king himself. As long as this personal loyalty was maintained, the machinery of centralised government in the provinces was kept absolutely minimal. Even the vital activities of tax collection and minting coinage were 'farmed out' to provincial elites. The issues with this approach can be seen in the fracturing of the Carolingian Empire and the territorial sqabbles which ensued following Charles' death. In contemporary Mercia, Offa's own attempts to establish a dynastic succession failed when his son was decided largely to not be up to the task. The hegemonic nature of the Mercian state, as essentially a federation of smaller client states, also led to great difficulties when the sub-kingdoms of East Anglia, Sussex and Kent allied and rebelled following the Battle of Ellendun in 825.

In contrast, Cerdicing Wessex was almost jealously centralised. The ability of the monarchy to develop its own authority was catalysed greatly by the developing Viking threat from the late 8th century onwards, which convinced not only nobles, but also previously rival kingdoms to work together in the face of an outside threat. The powers and extent of English monarchy are transformed and expanded largely by Alfred the Great in a process continued by his son Edward in Wessex and daughter Æthelflæd in Mercia and culminating in the unification of England by his grandson Æthelstan. The construction of the burh network and the legislation moving civic and economic functions under their aegis placed hubs of government activity throughout England from which centralised control could be maintained through the bureaucracy. Reticent nobles were warned of the dangers of their recidivism - Asser's Vita Ælfredi contains the cautionary tale of an ealdorman of Kent who did not maintain the necessary infrastructure of defence or follow the royal demands for garrisons and fortifications, and is killed inside his half-built walls by marauding Vikings.

Such systems have their limits. Such intense infrastructure was increasingly expensive to maintain, and control of England outside of Wessex was largely dependant on the co-operation of Mercia and Northumbria. Mercian co-operation was maintained largely through the skillful politicing of Æthelflæd and Æthelstan but hiccoughs did occasionally occur. Control over Northumbria was far more tenuous, and the region passes in and out of effective English control throughout the tenth century, with a period of particular open rebellion and conflict in the 940s. The matter is only resolved by Edgar in the 970s granting relative political autonomy in return for continued loyalty.

Such states can then fall to a rival, either through complacency or internal disquiet, or terrible mistakes on the part of their rulers. England is largely undone during the reign of Æthelred II because his relatively weak rule and long history of making very bad decisions allows for local nobility to openly challenge the crown and usurp local control. Such instances are similarly common in Carolingian France, where royal power often did not extend far outside of Paris. See, for example, the purely nominal loyalty of Normandy or Brittany in the 11th Century which would eventually boil over into outright rebellion following the Norman Conquest, and the division of large swathes of France under English rule in the 12th and 13th century. In Ottonian Germany, the defeat of Otto II at the Battle of Stilo in Italy caused huge ramifications. By over-reaching and suffering considerable losses, Otto II had to abandon his attempts to conquer Italy and was faced with renewed Danish attacks to the North and a mass Slavic uprising in his Eastern territories as the Imperial army was retreating in disarray through Northern Italy, setting back German Imperial ambitions by at least a decade.

E: Sorry this ended up slightly rambling. For further reading I would recommend:

Charles-Edwards, T., After Rome

Bartlett, R., The Making of Europe

Blackburn and Keynes, Kings, Currency and Alliances

Thietmar of Merseberg's Chronicon - a surprisingly personal and intimate Chronicle which pours over Otto II's campaigns in great detail.