The Complexity of Medieval Rule

 Here's a screenshot from the EU4 mod called "Voltaire's New Nightmare".

The purpose of that mod is to recreate the Holy Roman Empire with horrifying levels of detail. There are apparently several thousand provinces, each of which you can individually conquer.
This mod is so hard to run that to make it work the makers of the mod had to remove the rest of the world. (Hence the green areas around the HRE)
Here's a more close-up picture. Every different color signifies a different country.

And here's an even more close-up picture with one single province selected (thick yellow outline). Notice how the province is split into around a dozen different parts. This made me realize why borders in medieval times were so complicated. It all has to do with vassalage.

 

Nowadays countries are ruled through armies of bureaucrats. This is possible because throughout the world, there are well-established school systems, which produce staggering levels of bureaucratic talent, enabling whoever is in power to give out orders and expect them to be fulfilled with reasonable accuracy.
Not so in the medieval times.
In order to run a country, you need resources, nowadays usually understood as money. This is already a major issue - money wasn't used for the majority of life in medieval times. Taxation more often than not meant taking actual raw resources of the people you're taxing, transporting them to a central location and safeguarding them until you found some way to use them.
Furthermore, trade was less developed. If you wanted something that your subjects didn't produce, you'd have to ship those same raw resources to someone who wanted them, then ship whatever you were buying all the way back.
Therefore it was impractical to tax people who were very far away and it was better to spend resources directly rather than trade them. Since the main taxed good was food, that meant the best use for wealth was feeding people - such as those same bureaucrats, or perhaps soldiers.

But why the messy borders?

That has to do with how one would rule a country in times like these. It didn't happen through direct rule. A ruler did not tax all their subjects and therefore did not have access to most of their wealth. Instead, the ruler would pick out several people and hand them land, gaining in exchange a promise of loyalty - that the subject would help the lord fight, should the lord ask for it.
However, most of those subjects also couldn't rule their lands directly. The lands were simply too large, the management of them too complex and the transport too slow to even get everywhere.
Therefore most people who possessed land split it up and handed it out to their subjects, who in turn split it further and further for the same reason.
For example, a county was ruled by a count, whose subjects and their subjects would include many levels of people - barons, knights, clergymen, stewards.
At the lowest level of management, land might be parceled out as a single mansion or monastery. This single piece of land would have someone locally present to manage affairs. Likely one with an overlord of their own somewhere else, who they either pay in loyalty or goods to not loot them and protect them from others who would do so.
It is this low level of management that the last picture represents.

 

Even though in any reasonable sense, a province ought to be a contiguous piece of land, that was not necessarily how it was ruled. In any given count, different manors and monasteries might belong to different lords, hence creating borders like the ones seen above.
If things went well, they might remain in different hands for decades or perhaps centuries, until conquest or marriage united them. If things went less well, the death of some important guy somewhere might mean all his belongings being split between dozens of people and their subjects.
Alternatively, an army could come and loot the place so hard that no one cares who it belongs to for a while.

Medieval rule was complex. So complex in fact, that looking at it now, it seems like some kind of madness afflicted every single map-maker and forced them to randomly drip paint all over the place.
Even so, it, vassalage, was the best system they knew. It accounted well for human nature and was quite cheap to maintain. It seems crazy to us now only because we're used to a simpler world - one where governments don't usually loot their own people. One where you can acquire practically anything with money that's so effective it seems like magic. One where doing your taxes takes 5 minutes in front of a computer screen. Well, that last one only in truly developed countries, such as the one in which I live.