Why did napoleon’s reign make european leaders eager to restore the monarchies?

European leaders believed that having a royal family would go with traditions, and should be followed (especially because they interacted with each other through out the generations).
Napoleon brought radical changes to France, and other countries were afraid that what happens in France would happen in their country. Suppressing the French and returning the royal family would show the french and their own country what would happen if they try the same thing (again)

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