Having looked in part 1 at the causes of ethno-national unification in general, and the record on political unification vs. division within the Korean peninsula, let's see what forces have caused Korea to unify over history. We can then compare those forces to the current forces, in order to predict what will happen going forward. We'll start with the most recent period and work backward.
Joseon period (1400 - 1900)
The most recent unified Korean polity was controlled by the South, and lasted 500 years, roughly 1400 to 1900 (the Joseon kingdom). Recall that unification of one group builds in response to expansion by another, highly different group onto their territory.
It was the expansion of the Japanese into the Korean peninsula that largely drove the cohesion of the Joseon kingdom, with the southern region bearing more of the brunt and therefore leading the way in counter-Japanese cohesion.
Pirates from Japan began to harass southern Korea around 1350, and it got bad enough by the early 1400s that the nascent Joseon kingdom sent a naval expedition to wipe out the pirates' base on Tsushima Island, lying between Korea and the Japanese mainland.
That didn't stop them, and by the late 1500s, the whole peninsula was invaded by an army of over 150,000 commanded by the leader of a unified Japan. The northern part of Korea got back-up from Ming China, who knew they were next on the list if the Japanese took over Korea. That relegated most of the fighting to southern Korea, where civilian militias had to fill in for the lack of a large empire like China to help them out.
After a lull of peace, in the 1870s the Japanese used gunboat diplomacy to open up Korean ports to trade, with most of the targets lying in southern Korea. Things escalated until 1910 when Japan annexed Korea outright, and kept it until they lost WWII, when the Allied victors split it between themselves (Soviets in the North, Americans in the South).
Thus, from roughly 1350 to 1950, an expansionist Japan put incredible pressure on Korea, primarily where the peninsula lies closest to Japan. This made southern Korea more acutely aware than the north of the need to band together as one people, lest they be over-run by a people who were utterly alien to them.
The Japanese spoke a language belonging to a distinct family, their folk religion was unique, and their subsistence mode and growing civilization relied more on a maritime way of life -- making them seem, in the eyes of a sedentary Korean population, like a group of nomads, albeit ones who roamed by boat rather than by horse. They were like the Phoenicians or the Sea Peoples of the ancient Mediterranean who laid waste to sedentary agrarian societies.
During the Joseon period, the north did get invaded from Manchuria circa 1630, forcing the Koreans to pay tribute to the upstart Manchus of the Qing dynasty of China, and no longer to the Ming. Although this may have been a more humiliating single invasion than the Japanese occupation of a few decades earlier, the Manchus were not part of an ongoing expansion into Korea that was drawn out over many centuries, as the Japanese were.
As laid out in the first post, it is the "chronic" encroachments, rather than an "acute" strike, that cause the invaded group to slowly and steadily build its ethno-national cohesion. Even today, Koreans of either the North or South are more antipathetic toward Japan than toward China or Manchuria.
Goryeo period (900 - 1400)
Before the rise of the Japanese during the 14th century, the main threats to the Korean peninsula came from the north -- primarily the nomads from Central Asia and Manchuria, the most infamous being the Mongols.
These related groups spoke languages distinct from Korean or Chinese or Japanese, belonging to the Mongolic and Turkic families, which are similar to each other (whether due to descent from a common ancestor, or to contact-based sharing and influence). They shared their own religion, Tengrism, based on worship of the "eternal blue sky" that prevails across their territory, the Eurasian Steppe. In that environment, they followed a nomadic pastoralist way of life, herding livestock and traveling long distances by horse, not being tied down to the same plot of land to sow and reap agricultural crops.
That set up a meta-ethnic frontier between them and the people of northern Korea, separated by the Yalu River, the mountainous terrain, and the small area of the peninsula, all of which prevent Koreans from evolving into nomadic pastoralists.
Northeast Asian nomads had begun unifying during the second half of the first millennium, culminating in the Khitan Empire (or Liao dynasty) conquering much of northeast Asia from roughly 900 to 1100. Without missing a beat, another nomadic group took over -- the Jurchen, ancestors of the Manchus, who founded the Jin dynasty from roughly 1100 to 1200. They lacked the lands of Mongolia and further west that the Khitans had, but they still penned in Korea to its north and west. Then right on top of them, the Mongols broke out and invaded Korea over the middle part of the 13th century.
It was this chronic expansion of nomadic empires from northeast Asia that caused the Korean peninsula to unify during the Goryeo period. And because the threats were coming from the north, it was the more exposed northern Koreans who developed the strongest sense of solidarity needed to repel a common enemy, and they led the way in unification more than the southerners.
Ultimately the blow from the Mongols proved to be too much to withstand, and Goryeo was reduced to a vassal state of the Mongols (or the Yuan dynasty) from 1270 to around 1350. This weakened status of the northern-led kingdom, combined with the rise of Japanese threats from the south, led to the southward shift in the center of ethno-political gravity during the Joseon period that followed.
Today there appears to be less of a threat coming from Korea's north and west, as the nomads of the Steppe have gone into hibernation after the raids of the Mongols and Turks. However, those Central Asian Steppe explosions break out at least once a millennium, and it's already been 800 years since the Mongols. But those are not happening now, so they won't play a role in building pan-Korean solidarity in the meantime.
The other potential major encroachment from that direction would be China, and they don't seem interested in taking over Korea. Their grand vision is the One Belt One Road initiative to link most of the Eurasian landmass economically, and the Korean peninsula -- both North and South -- is nearly alone among East Asian countries in its exclusion from that plan. China is more worried about Southeast, South, Central, and West Asia. But given China's general plans of expansion, it's still a possibility that will make Koreans want to band together to repel.
Instead, it will likely be Japan that puts the most pressure on Korean ethnic solidarity and political unification. Japan still views itself as superior to Korea, and will continue to act on that, even when the American empire leaves both South Korea and Japan. And China's ongoing expansion is likely to make the Japanese cohere more strongly in the contest for regional dominance. The natural starting point for such a counter-Chinese expansion by Japan would be into Korea, where China has historically had little political control. Japan would start with an easier battlefield on which to challenge China for regional dominance.
Squeezed between China and Japan vying for control over East Asia, the Korean peninsula will likely unify rather than remain divided. And given the stronger pressure coming from Japan than from China, a unified Korea will likely be centered more toward the South, as it was during the Joseon period. The northern-centered Goryeo period came before the rise of the Japanese, when the only threats were to the north and west.
Unless the Japanese vanish as a regional power, it would take one hell of a chronic invasion from China or Central Asia over the Yalu River to make the northern part of Korea the leader of a unified peninsula.
The next post will briefly look at periods in Korean history when the peninsula was divided (before 900), to show that they are not applicable to the present day.