Nationalism in the 19th century was indeed a powerful, double-edged sword, acting as both a force for integration and disintegration across Europe and other parts of the world. Its core idea—that people united by common language, culture, or history should form a self-governing nation-state—had profound and often contradictory consequences.

In Europe, nationalism was a primary engine of integration. The most striking examples are the unification of Germany (1871) and Italy (1861). Here, disparate principalities and states, often sharing a common language and cultural heritage but fragmented politically, were forged into unified nation-states under nationalist leaders like Bismarck and Cavour. This process involved suppressing regional loyalties in favor of a larger national identity, consolidating power, and creating new, stronger political entities.

Conversely, nationalism simultaneously led to significant disintegration, particularly within multi-ethnic empires. The Austro-Hungarian Empire suffered immense internal pressure from burgeoning nationalisms of groups like the Hungarians, Czechs, Serbs, and Poles, each seeking their own independent state. Similarly, the Ottoman Empire, often dubbed the "Sick Man of Europe," witnessed the gradual secession and independence of Greece, Serbia, Bulgaria, and Romania throughout the century, as these ethnic groups asserted their distinct national identities against imperial rule. This process was often violent and destabilizing.

Beyond Europe, nationalism also played a complex role. In Latin America, after gaining independence from Spanish colonial rule, the formation of new republics often reflected an integrating nationalist sentiment that carved out distinct nations from a broader colonial entity. In India, the nascent Indian nationalism, though still developing, began to foster a sense of shared identity and purpose among diverse communities against British colonial rule, acting as a unifying force against an external oppressor.

Thus, 19th-century nationalism simultaneously built new nations from smaller units while tearing apart old empires, a testament to its revolutionary and often volatile power.