well the rise of nationalism led to world war one, but at the same time led to unified eastern european states, also allowed the german states to actually form a germany. nationalism formed the modern world, and broke up the hold of monarchy, broke down the barriers of large empires... And although not at the consequences were the best in hindsight, nationalist sentiment in places such as the balkans and in the russian empire fueled the course of history. American nationalism, while making us imperialistic, also made a unified american psyche that didnt exist before. Sure, people considered themselves American, but the American indentity, that spurns class or race, comes from nationalism. It allowed the disenfranchised, while still remaining disenfranchised, to feel like they were part of our country.