Previously, I alluded to the Balhae(발해,渤海) topic or the North-South States Period:
By the way, on a tangent here, for those Koreans, who believe in this newly created North-South States Period Theory or 남북시대 (신라+발해 = Korea), let me tell you -- it's pure rubbish, which I would like to address in detail one day(The main question behind that issue comes down to who were the Mohe (말갈, 靺鞨) people (A Schizophrenic Han : Breaking Down Borders: Korea).
Well, I'm not sure if it's "pure rubbish" anymore. A lot of information that is available on this topic is the subject of debate between many nationalist historians (you'd be hard pressed to find a better oxymoron), so much of what is available on the topic is unreliable at best. But, first, let me try to go over what is generally accepted. By the way, the Mohe (말갈, 靺鞨) people were ancient Manchurians.
What is generally accepted is that up until the late Joseon dynasty Koreans felt that Silla unified Korea for the first time in 664 A.D., but beginning at this point in time, certain Koreans felt that many Koreans had forgotten that Silla had unified only part of the peninsula and that vast tracts of Manchuria, which had been controlled or contested by Goguryeo and Balhae up until the end of the first millenium, had been largely ignored when studying the history and legacy of Korea.
What happened in 664 was that Silla in an alliance with the Tang Dynasty, whose forces Silla also had to defeat for control of the Korean peninsula, finally controlled the Korean peninsula by herself. In that the the (Early) Three Kingdoms Period of Goguryeo, Baekje, and Silla gave way to just Silla. But, sometime in the eighteenth century, certain Korean historians from the Joseon dynasty felt that the heritage left by Goguryeo had been ignored and that by ignoring vast tracts of Manchuria, which Goguryeo had controlled, but Silla couldn't, Koreans were indeed ignoring much of Korean history.
This is precisely where Balhae comes in. You see, those parts that Silla (or Tang) couldn't control ended up being part of a new state called Barhae(발해, 渤海). But, since most of Korean history from that point on comes from the Silla perspective or a Korea unified by Silla, studies of Barhae must come from either Chinese or Japanese sources. While much about Balhae remains controversial, I'd say the following statement is less so. This is from an old Japanese source that recorded a letter from a Balhae king:
'復高麗之舊居,有扶餘之遺俗' translated into "Balhae is the country that upholded the tradition and spirit of the Koguryo and Buyeo" (續日本紀; English translation found from The Chinese History Forum).
So, in other words, Balhae, according to those who lived in Balhae as the Japanese source reports, is the successor state to Goguryeo. But, what does this mean? Does this mean that the kings and people of Balhae are indeed of Goguryeo (or Korean) stock? And, what does it mean to be Korean? Does living under Goguryeo mean that you are indeed Korean? Or, no? This is where it gets complicated. For me, I was under the impression and still am under the impression that the kings of Balhae were probably of Goguryeo(Koguryo, Buyeo) stock, but it's people less so. I'll get into what Buyeo means in a second. But, first:
The largest "ondol" heating system dating from the Balhae Kingdom has been unearthed in a nearly intact state in Russia's Maritime Province, confirming the kingdom to have been a Korean settlement.
Ondol, literally "warm stone", is an under-floor heating system where flues carry hot gases below the living space. They were a distinct feature of Korean dwellings and are not found in the remains of Chinese, Khitan or Jurchen homes ("Heating System Confirms Ancient Kingdom Was Korean" : Chosun Ilbo).
This makes it seem as if not only the rulers were Korean, but that the people who lived under Balhae were also Korean (Khitan or Jurchen are names for Manchurians or ancient Manchurians). Ironically, what is interesting is that I would actually argue that from what I have read is that Japan should be seen as the natural successors to Goguryeo (haha, I might get shot here), but those that spoke a Buyeo language were Goguryeo rulers(and subjects), Baekje rulers (and possibly her subjects), and, yes, Wa (Japan). Notice the conspicuous absence of Silla here. As Silla unified the peninsula, Buyeo became an extinct language on the Korean peninsula.
There is actually a stone edifice found with Goguryeo inscriptions during the Japanese colonial period, which led Japanese scholars at that time to believe that Japan had colonized Korea before. This, of course, is preposterous as it is actually the case that "Korean" (or Buyeo-speaking) people from the peninsula were fleeing from Silla that colonized Japan and that those that had fled to the island had actually returned back to the motherland.
At any rate, I'm not yet sure how much this new discovery lends credence to the case that Balhae was a Korean kingdom, but if only Koreans used the Ondol heating system and those living under Goguryeo used it and those that lived under Balhae used it, but the people that lived under
Jinthe Liao Dynasty and later Manchurian states did not in the same geographical area, I would be inclined to think that Balhae was indeed a "Korean" kingdom.
Of course, this gains additional traction today when you consider that Korea remains a divided country with there being a northern Korean state and a southern Korean state and that this would then resemble the period where there were two Korean states a millenia ago (Balhae and Silla). So, perhaps, (Silla + Balhae = Korea, 신라+발해 = 코리아) as much as (North Korea + South Korea = Korea).