Preface this by saying I'm mostly thinking about England and (northern?) France in the early-mid fourteenth centuries, but any medieval place and period is welcome!
So with modern states, we're accustomed to thinking of them in terms of borders and area -a state rules the territory within these defined bounds on the landscape, where it has a monopoly on force. We don't think of space between states, and we identify the highest level of government with the country itself, and associate with it supreme power. Even when the subdivisions of a modern state have significant autonomy, we tend to think of them as purely domestic actors, without any external voice or power.
I have a feeling this model is highly inappropriate for the middle ages, but I want to make sure how I am conceptualizing it isn't totally groundless. Rather than defined territorial kingdoms, it's more appropriate to think of (sacral) kingship, which function as centers to which lords and princes -cumulatively much more powerful than any king- in the spaces between the centers of royal prestige and power can attach and detach themselves, based on constant negotiation, renegotiation, and force. Even when nominally subordinate to a royal center, these lordships and principalities would retain independent prerogatives for external matters.
This is sort of an ideal-type conception, and I don't mean it to sound too systematic and defined, but am I thinking about this right?
When talking about England and France in the mid -14th century, it is important to understand that we are talking about a very tumultuous era which saw radical state change - the first and second phases of the Hundred Years War, the reign of Edward III in England and the beginning of the Valois in France, and towards the end of 1360s we see Charles V come to power in France and rapidly transform the fortunes of the French kingdom. This is all as well as the Black Death.
This lead to a situation where both realms had to rapidly expand usable military fortifications, had to procure funding, had to deal with multiple major epidemics and destructive periods of war leaving large areas of their countries ravaged and ruined, displacing people and killing millions. Certain regions, especially in what is now France, became extremely depopulated. To make matters just that little bit worse, economic woes were severe, especially in northern France and Flanders.
Also, an important consideration for this period is that England and France had very different political conditions and systems of governance. England in the 1330s was by all measures one of the most centralised states in all of Europe, and certainly in Atlantic Europe. England was still a kingship ruled by a dynasty who held other lands, and interacted with those lands in terms of property - but the English nobility, as a total, had both considerably more voice in central matters of state (under Edward III) and less ability (and motive) to seriously oppose Edward III by military means than the French nobility did in France. Large parts of the 'kingdom of France' were functionally independent more or less, from the duchies of Brittany and Burgundy, to the smaller but still powerful noble families in the southwest. The changes of loyalty of some of these Southwestern noble families in Bearn had serious impacts on the rest of France's military fortunes. There is no analogous situation in England, across the channel, and it is one of the many reasons that England was able to fight such a devastating war against a much larger, and wealthier, opponent for so long. On multiple occasions in the Hundred Years War - it was French civil conflict and instability that was England's greatest strength.
All this combines to making this specific time and place probably one of the biggest grey areas in the question of 'state development. Both 'states' in question functioned very differently when we analyse them on a very close basis - on such matters as taxation, governance, military means and the lengths to which we can describe one or the other as having a multifacted 'government' - they both are very different. To make matters complex, both realms are in multiple major conflicts. There is not merely the very severe Hundred Years War, but a French Civil War, Anglo-Scottish wars, and a major rebellion in Wales. The Hundred Years war even outflows to include Navarre, Castille, Aragon, Genoa and Portugal. Safe to say, it gets out of hand and campaign season to campaign season there can be very major changes in policies.
I'd argue, in this period it is reasonable to think of the English kingdom as a state or, more accurately, a "proto-state" in the modern sense - with defined boundaries, governance and a rapidly developing and expanding system of taxation and economic policy (the English even institute specific product embargoes and ship taxes, a not-small endeavour for a 14th century kingdom) and growingly, a significant involvement and interest of people of multiple classes and means in the betterment of 'English' interests and afffairs.
In France, large changes in this respect occur throughout the war. Towards the end of the Hundred Years War, in the 15th century, the French crown institutes a standing army (!), greatly expands and improves state function, takes further direct control over areas previously 'governed by hear-say' more or less, and as a result, grow to not just meet, but overwhelm the challenges that the English can militarily bring to bear against them. However, it is likewise true to say that France, for much of the earlier phases of the conflict - excluding maybe the reign of Charles V - was in no way as centralised or capable of major military or economic mobilisation and endeavour as the English kingdom.
we identify the highest level of government with the country itself, and associate with it supreme power. Even when the subdivisions of a modern state have significant autonomy, we tend to think of them as purely domestic actors, without any external voice or power.
This is, for much of the middle ages and in many places - a very good insight. You'd be correct in asserting that for much of the middle ages, particularly in France but also elsewhere - that many 'subdivisions' held significant independent power and acted largely of their own accord. In this respect, for nearly all of French medieval history - up until the tail end of the Hundred Years War - thinking of the French kingdom as a unified state or area of government is wrong.