Ahead of a high-profile, first-ever summit between the leaders of the US and North Korea, there is a lot of attention being given to the individuals involved, exacerbating the tendency to focus on personal rather than societal factors. So let's zoom out and see what the historical background and the current context suggest about how the relations will play out.
This look will be broken up into several posts, starting with the forces that cause national unification over the time-scale of centuries, and ending with the forces that cause paradigm shifts in political regimes over the time-scale of decades. After all, a unified Korean peninsula would certainly require paradigm shifts in both the North, South, and the US, compared to the status quo of the Cold War.
Beginning with the conclusion, I think when the American empire leaves the Korean peninsula, it will in fact unify, to become as strong as possible under the pressures of both China and especially Japan (and a fat-tail risk from Islamic radicalized nomads from Central Asia). The state will be centered more toward the south than the north, as southerners will feel greater pressure to band together in the face of threats from Japan, while northerners will feel relatively less pressure from their north.
However, that may have to wait until paradigm shifts occur in the regimes of the major players -- already there in the South, perhaps soon in the North, but not yet in the US. Still, the process should be observably under way within the next couple decades, mainly waiting on a change of regimes in the US -- from Reaganism to Bernie-ism, as Tulsi Gabbard takes Mike Pompeo's place as Secretary of State.
Brief history of unification and division in the Korean peninsula
First, we emphasize that a unified polity in Korea is not a given, over the region's history -- nor that a unified peninsula would be centered toward the south.
It was under unified control for the last thousand years, from roughly 900 to 1400 (Goryeo kingdom, centered toward the north) and roughly 1400 to 1900 (Joseon kingdom, centered toward the south), but not for the thousand or so years before then. From roughly 1900 to 1950, it was occupied by Japan. After Japan lost in WWII, it was split between the major victors, with the Soviets supporting the North, and the US the South.
During the roughly 200 years before unification began, the peninsula was divided between northern and southern kingdoms (Balhae and Silla), with the north having a greater record of expanding outside its natural boundaries (e.g. into Manchuria). For the roughly 700 years before that -- back to the start of the first millennium -- there were in fact three kingdoms, with the southern region being sub-divided into two, and one of them (Baekje in the southwest) being an off-shoot of the northern kingdom (Goguryeo). Even before the Three Kingdoms period, there was one kingdom in the north (Gojoseon) and a confederacy of states in the south (Jin).
Over the millennia, the boundaries between north and south -- when the peninsula has been divided -- have remained close to what they are today.
The point is that the current division is not necessarily an artificial status imposed by external powers, as though the peninsula had an inherent tendency to be unified. For awhile it was, and for awhile before that, it wasn't. Should it remain divided long after the US pulls out, it will not necessarily be due to a "legacy of imperialism". For the longest time, the peninsula was in a stable state of division along roughly the current north-south lines, and it's possible (though unlikely) that it will return to that stable division for whatever reasons kept it stably divided before 900 AD.
And if it does unify, it's not inherently going to be centered toward the south, since during the two equally long periods when it was unified, one was more northern and the other more southern. We have to analyze what made unification more northern or more southern in the past, and see which of those sets of forces is the closest to the current set of forces.
Causes of national unification
That brings us to the matter of what forces cause smaller polities to merge into larger ones, from a loose tribal confederation up to a multinational empire.
There must be a "meta-ethnic frontier" dividing two very different sides -- different language, religion, subsistence pattern (farming vs. herding), physical appearance (bodily as well as adornment), historical territory, and so on. This sets up the strong sense of "us vs. them".
One of those sides must be already unified and expanding, so that it encroaches upon the other side. When one side is highly unified and literally moving in your direction, your own side had better unify its various little groups in order to withstand the advance, repel it back to where it came from, and maybe even conquer them in turn.
The advance by the other side must last over a long period of time, rather than be a fluke. If it were only a one-off "acute" encounter, you deal with it as best you can but don't bother changing your societal structure long-term. If, however, the encounters are "chronic," you had better change society to deal with it. That means that unification will be slow and steady, rather than a rapid response, as the people on the receiving end of the expansion are trying to figure out if they are dealing with an acute or a chronic problem.
Ethno-political unification shows hysteresis because it is not cheap or easy to do, given all the usual competing interests across the small-scale groups and among the individuals involved. It is not like flicking on a light switch whenever you enter a room and need to see.
When a process is very costly and difficult to kick into gear, it will want to stay turned on and "idle" when there is an apparent lull in the reason that it turned on in the first place. That's why you don't turn off your car at every red light you come to, and then turn it back on again when it changes from red to green. The process only winds down when there is a sustained absence, rather than just a lull, in the reason for turning it on.
Peter Turchin lays out the social mechanics of this process in technical and popular books (Historical Dynamics, War and Peace and War), illustrated with numerous examples across time and geography.
To give one example, the Romans unified into larger and larger groups due to the expansion of the Celts from the butter region of Europe into the olive oil region. Eventually the Romans became a unified expansionist nation of their own, and turned the tables on the Celts during the Gallic Wars. After a sustained absence of the original Celtic threat, the Roman Empire began losing its raison d'etre, its elites began in-fighting over status rather than banding together against a common foe, and the Romans devolved back into a smaller-scale group.
Having set up this basic framework, the next post will look at the expansions into the Korean peninsula that have caused it to unify before, so that we can compare them to current or likely expansions against the peninsula, to predict where the situation goes from here.