Before the end of the Cold War, North Korea was a Soviet bloc country and Soviet bloc countries were closed off from the West, but not from each other. Before the fall of the Iron Curtain, North Korean students were permitted to attend universities in the U.S.S.R. and communist eastern Europe. Essentially, North Korea has kept the old Soviet-style isolation going, just without there being a Soviet Union to be in isolation with.

Before the Cold War, there was no such thing as North Korea. Like Germany, Korea was divided in half after World War II, with North Korea founded in 1948 as a Soviet client state. Prior to that time, the two Koreas were one. Nevertheless, pre-20th century Korea (comprising what's now North and South Korea) was famously isolationist and was nicknamed "the hermit kingdom".

It appears that the other posters have been tip toeing around Japan's occupation of Korea, and I want to just chime in on this particular aspect.

Japan assumed control of all Korea after an annexation treaty in 1904. They immediately started imposing several Japanese institutions, including public education, administrative systems, public health, and the like, although they did it in a way so as to ensure the Koreans would remain fully aware that they were a) subordinate to the Japanese and b) that the Japanese rule was justified.

In North Korea in particular, Japan saw it as both an economic boon as well as a passage to Manchuria as well as the Soviet Union. In addition to economic development efforts (where essentially Japanese companies would extract resources using Korean labor, primarily iron and lumber from North Korea), the key developments were railways from Korea into Manchuria and thus the rest of China as well as through the Trans-Siberian railway to Europe. This gave Japan an avenue for international trade that was not dependent on shipping. To protect this, Japan also created a garrison army called the Chosun army which had several defense posts along the Korean border with Manchuria.

Like the rest of Korea, North Korea was subject to vacillating Japanese policy priorities, from the initial pure imperialism of the early years, to stomping out political descent, to gradual liberalization and opening up in the Taisho democracy, to full on attempted assimilation with the rise of militarists in the 1930s.

I've mainly studied the Japanese colonial period and modern North Korean history and it seems like quite a bit has been covered here, so I'll just add a few things I know to what everyone else has said.

Northern Korea, especially in Pyeongyang's own Pyeong'an province, was one of the more Christian regions of Korea during the 20th century. Pyeongyang was considered the center of Christianity in Korea and before the division of the peninsula was called "Jerusalem of the East". Due to this, it was a base for many western missionaries and was also a large political center for Christian nationalists in Korea's independence movement. In fact, one of the most prominent Christian nationalist leaders, Cho Man-sik, ended up founding one of the minor political parties in North Korea after liberation and was even asked by the Soviets to be the head of the new government. In addition to what has already been said about the colonial period, the Japanese largely followed uneven colonial economic models, developing more heavy industry in the north and focusing more on agricultural development in the south. This development actually ended up in allowing post-division North Korea to be more fully economically and industrially developed than South Korea until the 1970s.

Before the 20th century, northern Korea has been an important region too. When the Han dynasty defeated Wiman Joseon, the Lelang commandery they set up had Pyeongyang as its capital. Later, the Korean peninsula was separated into three kingdoms, with Goguryeo encompassing northern Korea as well as parts of Manchuria (though the Chinese government will try to argue that this was not a Korean kingdom). As well, North Korea's city of Kaesong used to be the capital of the later kingdom of Goryeo.

If you're interested, here's a good collection of photos of Pyeongyang during the Japanese occupation of you want to see what the city looked like before it was destroyed in the Korean War. Some buildings are clearly Japanese, but a lot have been around a lot longer. http://blog.naver.com/seihwanny/220063681777

Yeah, everyone here except for a couple keep skipping over a very, very important period in Korean history that shaped modern Korean identity, both North and South-Japanese colonial rule (1910-1945). (and Korea was colonized, not "occupied").

Under the Japanese colonial government during the 1930s, the northern half of Korea was heavily industrialized mostly because it was Japan's gateway to the empire's most important possession: Manchuria. This led to all sorts of inadvertent socio-economic transformations that carried over into the postcolonial period. What some scholars call "colonial modernity."

So despite the devastation of the Korean War, enough of the infrastructure and human capital in northern Korea survived to allow the DPRK to actually be economically better off more than the ROK, at least until the1970s. Massive Soviet foreign aid helped too. This bolstered Kim Il-Sung's legitimacy, though North Korean state was always pretty ruthless towards dissenters. By the way, North Korean repression was probably influenced by both Stalinist practices and Japanese colonial police measures to silence dissent.

(South Korea's later economic boom also indirectly drew from colonial experiences but in a different way).

More importantly perhaps, the Kim dynasty's REAL claim to legitimacy is Kim Il-Sung being the only real Korean military figure fighting the Japanese during colonial times. Now in reality Kim was only a minor irritant to the Japanese; he mostly launched guerilla raids from Manchuria into Korea. But thus the legend was born.

North Korea has had a bizarre strange, almost obsessive relationship with Japan (to be fair, it's a little vice-versa too) Japan is always villified along with South Korea and the US, though often in more strident terms. There was the whole kidnappings of Japanese citizens by North Korean agents a few decades ago. And yet, one of the strongest overseas pro-North communities are DPRK-affiiated Korean residents in Japan (another legacy of colonialism). Kim Jong -Il adored sushi and had a Japanese chef for years. He also fell in love with a Japanese-Korean woman who became the mother of North Korea's current leader-Kim Jong-Un.