Nationalism, a people’s sense that they belong together as a nation because of a shared history and culture, and often because of a common language and/or religion, emerged at the close of the Middle Ages (500–1350). By the 1700s several countries, notably England, France, and Spain, had developed as “nation-states,” groups of people with a shared background who occupy a land that is governed independently. As political and economic entities, the nation-states were preceded by fiefs, tribes, city-states, and empires, which overlapped each other as organizing units, dividing peoples’ loyalties.

By the 1800s nationalism had become a powerful force, and the view took hold that any national group has the right to form its own state. Because of this belief, called national self-determination, some nations achieved independence (including Greece, which gained freedom from Turkey in 1829, and Belgium, which won self-rule from the Netherlands in 1830); others formed new, larger countries (both Italy and Germany were created by the unification of numerous smaller states, Italy in 1870 and Germany one year later); and still others carved smaller states out of great empires (for example, the breakup of the Austro-Hungarian Empire following World War I [1914–18] resulted in the formation of the independent countries of Austria, Hungary, Czechoslovakia, Poland, and, later, Yugoslavia).

In the United States, nationalism in the 1800s took the form of Manifest Destiny, the mission to expand the country’s boundaries to include as much of North America as possible. By the end of the century the United States had claimed all of its present-day territory.

While nationalism is a source of pride and patriotism and has had many positive results, some leaders (notably German dictator Adolf Hitler [1889–1945]) have carried it to extremes, initiating large-scale movements that resulted in the persecution of other peoples and in the hideous practice of ethnic cleansing.

The boundary lines that were drawn on the world map at the end of the twentieth century were largely the result of nationalistic movements, some of which had resulted in conflicts—and some of which remained unresolved.