Note that France was united around the first decade of the sixteenth century (I believe), Spain - 1492, Germany & Italy - 1870, Russia - I believe some time in between - I think most of what is Russia today was occupied by remnants of the Golden Horde (or Mongols) until the 16th or 17th centuries... So, you can imagine why Korea could be so nationalistic and so xenophobic (not a single minority ethnic group). I believe Korea and Japan are the only two countries, where you'll find almost no ethnic Chinese communities in significant numbers in all of East Asia. Up until recently, I believe there were 50,000 ethnic Chinese out of a population of almost 50 million (about 0.1%).
In this context, the issue of whether these kingdoms, such as Goguryeo (高句麗) or Balhae (渤海) were "Korean" are rather unimportant and, perhaps, even irrelevant next to how it is the merger of the separate kingdoms' people and culture (for more: check out Samhan/Buyeo, 三韓/夫餘 - in Sino-Korean/扶餘 - in traditional Chinese script) that gave rise to the singular identity of a Korean people. This is probably the most important theme to take from ancient Korean history (along with the geography of the Korean peninsula and the geographical location of the Korean peninsula). Also, I mentioned the Gangnido Map (and how it has a really long name. Wikipedia cites the term, "Honil Gangni Yeokdae Gukdo Jido ("Map of Integrated Lands and Regions of Historical Countries and Capitals", 혼일강리역대국도지도, 混一疆理歷代國都之圖) for the map. Nonetheless, it is an easy and quick way to visualize how Koreans saw themselves in relation to the outside world during the Joseon dynasty.
Interestingly, in China the latter term was not used until after normalization of ties with South Korea. The term Hanguk actually derives from the period 1897-1905, when the Joseon dynasty fully joined the Westphalian (Western/Modern) International Order and which we have not yet discussed -- when it was no longer considered a vassal state after the first Sino-Japanese War (Became, the Great Empire of the Han People (not to be confused with the term Han used for Han Chinese (漢) vs Han (韓) Koreans)). On a side note, this is precisely the reason why classical Chinese characters are important as in both the Korean and Japanese languages there are a large number of homonyms that can't be described by words constructed from an alphabet (this is precisely why Kanji (literally Chinese Script or Sino-Japanese characters) are still in heavy usage in Japan and many (should) be in usage in the Koreas today...I believe -- there are 5 vowels in the Japanese language and no differences made through pitch. Korean is much the same except with a larger number of vowels and for the most part a single pitch as well).
The historical connection between the Joseon Dynasty (the Hermit Kingdom, Land of the Morning Calm) and North Korea becomes very straight forward when thought in this light (and as we shall see over the rest of the semester).