Most of Korean, Japanese and China have got brain washing about nationalism. It still happen in their elementary schools.
Their basic idea about nationalism is everybody is one family long time ago. So they need to help each other against the other nations. In Korea we have one origin ancestor, Dangun, BC 2333. According to the Dangun legend, every Korean has one lineage. North Korea emphasis on the nationalism more than South Korea, in order to sustain their system. A few years ago, North Korea claimed they have found Dangun's tomb.One thing which is very contradict is that according to the Dangun legend every body are one family. Now Korean has strong idea that every Korean in history had shared the Dangun legend after BC 2333. If it is true, then 500 years ago a Korean slave could insist my ancestor is same with my owner's ancestor. But it is obviously true that the hierarchy system in Korea history based on the different linage idea; A noble's blood is totally different from that of slave. If a slave had insisted it, he might be executed in public. One idea refers everybody is one family, and another idea refers we have different ancestor. How could it happen?
The Dangun legend is a replica of 'TenO' legend in Japan, which means King from Heaven. When the japan was to face to fight against China, they needed more support from their people, about 1900. They have developed their nationalism. According to the idea, their nation is selected by heaven and they need to expand their nation. And some of Korean have learned Nationalism from them when they adopted many western cultures; Most of cases Korea had imported western cultures from Japan indirectly.
Nationalism is newly developed idea even in western. But most of people believe it has inherited from very very old history. However, at most it has only about 400 years history in Europe. And in Japan, Korea, and China it is at most only 100 years.