How did nationalists threaten the borders set up by European monarchs?

Nationalists threatened the borders set up by European monarchs by promoting the idea of self-determination, where ethnic and cultural groups sought to establish their own independent nation-states. This led to demands for the redrawing of borders based on national identity rather than the dynastic and territorial claims of monarchs. As nationalist movements gained momentum, they often resulted in conflicts, revolutions, and the fragmentation of empires, challenging the established political order. Ultimately, the rise of nationalism contributed to significant geopolitical changes in Europe, particularly during the 19th and early 20th centuries.