The answer is Nationalism. Nationalism is a political ideology that states that one's nation is unique and, in an extremist form, it states that one's nation is the most important of all nations. This ideology emerged as a modern political system of ideas that tends to link people who share a common language, a series of symbols and a unique culture in a determined territory in order to create a modern state. That is why it was crucial for the creation of today's nation-states and many times, mainly during the first half of the 20th century, it was exacerbated and thus gave place to the creation of the most extremist and intolerant nationalist ideologies of History like Fascism and Nazism.