This is a topic I touched on yesterday here. As the Roman imperial system fragments and crumbles, it takes with it the widespread logistic and economic networks which allowed much of the Empire to flourish. Under the Empire, the widespread trade and distribution networks had allowed provinces to focus their agricultural efforts on cash crops and prestige items, in the knowledge that food could usually be imported. Rome's constant wars ensured a steady stream of slaves to maintain the vast pool of servile labour which enabled the everyday function of vast villa estates and major infrastructure projects. Security was provided by Imperial garrisons, wars were waged by the Imperial army paid by Imperial coffers, and bureaucracy could be centralised.

When the Empire crumbles and Imperial bureaucracy and troops are withdrawn, the provinces are left to fend for themselves. Some cope better than others - nascent Merovingian Francia manages to preserve elements of Imperial bureaucracy, infrastructure and governance, for example, while Gildas provides a heartbreaking portrait of the struggles and eventual failure of Romano-British authorities to try and hold things together as Rome leaves them in the lurch.

Without Imperial support, infrastructure becomes increasingly hard to build and maintain, and large, specialised estates are abandoned for a return to subsistance agriculture. Armies have to be comprised of peasants who are also needed in the fields, and as a result, regional elites find out just how expensive wars can become. Without regular correspondence with Rome, governance falls back on provincial structures. In provinces like Britannia, this is compounded by the fact that incoming Germanic peoples are largely illiterate, with no written laws, administration or advanced governmental structures.

In the void left by Rome, societies across the former Empire fall back on their pre-Roman structures, usually at a tribal level. In provinces where a semblance of Imperial order and, crucially, the Church survive, nascent states are able to enforce a certain level of widespread control, but this is still based on ethnographic lines - without a Romanitas uniting them, the Merovingians can establish control over Francia, but gain little traction over the Anglo-Saxons, Lombards or Visigoths around them. The Church is central to this. The theology established in Augustine of Hippo's City of God posits the Church as the spiritual successor to the Roman Empire, charged with the duty of maintaining peace, order and unity throughout Christendom. In the face of such widespread collapse, however, it is often all that the Church can manage to simply hold things together.

In former provinces like Anglo-Saxon England, a Germanic tribal structure flourishes in a world without an overarching bureaucratic oversight. Without standing armies and appointed governors, heirarchies based around personal loyalty supported by gift giving and feasting allow warlords to establish numerous petty kingdoms based on the territory that they and their retinues can immediately physically control, supported by an agricultural basis of immediate support. It is only after the arrival of the Augustinian mission to Kent in 597 that the reintroduction of the Church and a literate bureaucracy allows for the steady expansion of royal authority beyond petty kingdoms.

As I mentioned in my earlier answer, as kingdoms increase, some fervently centralise their power and others allow for a heavy degree of decentralised power. Centralising a kingdom requires a vast and often very expensive bureaucratic presence. In Cerdicing England, this is maintained through the burghal network. This network of centralised sites allows for the everyday maintenance of the kingdom to be carried out by royal agents while the King progresses around the country to maintain his rule and carry out his duties on a local level. In Carolingian Francia, however, loyalty is largely established on a personal level, and regional elites are often left to their own devices in return for their continued support of the king - a far cheaper but much more parlous system.