Although I can't speak in great detail for Europe, in East Asia nationalism was often a simultaneously occurring process with/result of post-colonial movements. In China, the collapse of the last imperial dynasty in 1911 was directly followed with a surge in nationalist movements that emphasized the need for constructing a distinct Chinese identity and building a stronger nation-state to withstand the forces of Western and Japanese imperialism. Chinese nationalism exploded in 1919 following the Treaty of Versailles, which gave Qingdao, a former German colony, to the Empire of Japan rather than returning it to Chinese rule. Students protested in the street and most of China's most important political leaders of the 20th century collectively recalled it as their formative political moment.

Similarly, Korean nationalism swelled with the annexation of Korea by the Empire of Japan in 1910. Tensions also exploded in Korea in 1919, when student demonstrators protested the increasingly violent and oppressive rule of the Japanese.

In general, nationalism only seems to be on the rise in East Asia as well, with countries like Taiwan and regions like Hong Kong butting heads with the Chinese mainland over what they perceive to be the erasure of their ethnonationalities and sovereignty. That's not to mention all of the islands and seas that are disputed by each nation-state, and the legacy of the Empire of Japan during the WWs. I have lived and studied in Japan, Taiwan, and mainland China (where I live now) and it can get very tricky sometimes navigating all of that political and historical baggage!

Sorry that I don't know as much about Europe but these nationalist histories are all connected, I think, as historical notions such as national identity and ethnicity are all constructed with reference to those that are within or outside of the group in question. In the case of East Asia, Western European and Japanese imperialism were essential parts in forging the boundaries of identities such as "Chinese" or "Korean." Hope that helps a little!