How can nationalism be both a unifying and a disunifying force

nationalism is a unifying force when people being united are all similar in race, culture, religion, language, etc. It brings the people who have things in common together. Nationalism is dis-unifying when people who are not similar are united under a single nation. Differences can keep people apart and can only be overcome by destroying their former institutions and principles which made them a distinct and strong group of people (race, culture, religion, language, etc.) and molding a new one.

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answer; taking up arms to defend the nation;