THE RISE OF THE TERRITORIAL STATE

"One (state) that is too large is capable of self-defence in what is necessary; but then it is a nation: for it will be very difficult to accommodate a form of government to it: for who would choose to be the general of such an unwieldy multitude, or who could be their herald but a stentor?"
Aristotle, unwisely objecting against the idea of Flächenstaat. Note the use of "nation": is he referring to a territorial state, to a large people bound by blood (instead of by a government) (id est, the concept of ethnos or ethnic group often translated as "nation", for instance in the KJV), or both? He is actually referring to a large people bound by blood, but not by government (the concept of ethnos), moreover, prone to anarchy due to lack of government. Just like the vernacular usage of "nation" as synonymous with hinterland state (territorial state) or Flächenstaat.

In the Middle Ages, Saint Thomas Aquinas weighed the pros and cons of the Flächenstaat, which he called "province" ("provincia"). He said that, on one hand, (agreeing with Aristotle), such a vast expanse of land would be very hard to rule. And that, on the flip side, a "province," the larger it was, the more difficult to be invaded by enemy armies (consider, for instance, the case of Russia, the largest state on Earth, 80% or 90% hinterland, which has successfully withstood Swedish, French, Japanese, and German invasions thanks to its redoubtable size among other factors). Long story short, he said the same things as Aristotle, but putting more emphasis on the military potential of the "nation"/"province" (difficulty to invade and capability of self-defense) than on the difficulties of ruling a large country with a numerous population. Was he being prophetic, even a little?

Still in the Middle Ages, later on...
When a certain alchemist friar brought about a sudden, devastating explosion in his lab, little did he know about the revolution he had unleashed. Many decades later, Renaissance rulers, to reaffirm their power against defiant feudal lords, brought cannons against the walls of their insufficiently guarded castles, fortresses, and holdfasts. The triumph of the Crown in all those lands was both total and quite obvious, expanding the hinterland of the kingdoms to heights not seen since the Age of Empires, centuries ago (The same happened in Westeros, although, instead of cannons, for rule of cool, there were dragons... Well, actually, the first cannons in the West were dragon-shaped, both to scare the enemy and for rule of cool!).
In other words, cannons brought about the expansion of the state hinterland and the establishment of the Flächenstaat. As a result, confident royals could move their courts to the outskirts of their capitals, with larger palaces and elegant gardens; while the ends of the realm were secured by militarized and fortified (with star-shaped fortresses and lots of cannons) outpost communities: these were the world's Küstrins, the Saint-Jean-de-Luzes, the "frontier towns" so often heard of in fairytales. The establishment of a professional officer class (instead of mercenaries), with the rank system so familiar to us nowadays (ensign, lieutenant, captain, and so on), was as much in consonance with the spirit of warfare that pervaded the West as the rise of firearms, of the territorial state, and of outpost communities (the latter two, like the professional hierarchized officer class, had already existed in the empires of yore), as well as the rise of diplomacy and inter-realm alliances. Like the one who has seen the storm clouds from afar and puts on a raincoat and umbrella, every realm was all geared up for both offensive and defensive warfare. This panorama of militarism echoes the similar "armed calm before the storm" that preceded the World Wars during the Belle Époque. The realms of the West were, then, also preparing up to the teeth for the upcoming and all-pervading storm of war. For an all-pervading storm of war that would last Thirty Years, and that would shape the human geography of the world we now know.

For more information, I advise you to consult:

The Shield of Achilles: War, Peace, and the Course of History, by Philip Bobbitt.

Recommended because of its exploration of developments in warfare and the early modern rebirth of the territorial state/nation/Flächenstaat as concepts that go hand in hand.