Idealism as a basis for nationalism or anything else is potentially dangerous because there is little or no reality check. Thus, things can be manipulated to extremes, by for example the unscrupulous or deluded. Typical consequences include jingoism (aggressive or warlike foreign policy), xenophobia intense or irrational dislike or fear of people from other countries) and ideologically based nationalism (a passionate feeling of superiority over other countries). EU nationalism is obviously based on idealism which, when it suits itself, ignores reality.
https://vk.com/video503532632_456240772
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