As others have noted, nationalism was a major cause of World I. Nationalism, a powerful force in the nineteenth century, was rooted in two dominant intellectual movements that were otherwise often at odds: the Enlightenment and Romanticism. The central Enlightenment urge to classify all things, including people, into groups, led to the rise of the concept of people grouped not by the boundaries of their nation state, but by shared language, culture, and ethnicity. Romanticism, with its focus on the common man and the exotic, shone a light on the customs of far-flung ethnic people and cast their struggles for freedom in an exalted, very positive light, fueling their movement.

By the beginning of the twentieth century, Balkan nationalism was a significant force. In order to take advantage of the weakness of the fading Ottoman Empire, Greece, Serbia, Bulgaria, and Montenegro—all part of The Austro-Hungarian Empire—formed the Balkan League in 1912 to fight the Ottomans. This led to a rise in Pan-Slavic pride and an increased hunger among these nations, especially Serbia, for independence from the oppressive Austro-Hungarian Empire.

Nationalist stirrings during the war caused problems for superpowers like England, who were feeling enormous economic strain to finance a costly struggle and did not have the time, energy, or resources to get involved in suppressing rebellions in their own territories. The Irish rose up during the war, eventually achieving independence shortly after the war's end. India also threatened to rise up, but was bought off with what proved to be false promises of being helped to independence after the war.

As we know, the Serbian assassination of Archduke Franz Ferdinand was the spark that ignited World War I, but nationalism didn't stop there. During the war, different Slavic regions, such as Czechoslovakia, formed legions to fight on the side of the French in return for the promise that if the Allied won the war, France would help countries like Czechoslovakia set up independent states. This shows that nationalism had a direct impact on the fighting of the war itself. After the war, the Allies were instrumental in supporting the creation of independent Slavic nations out of what had been the Austro-Hungarian Empire, also giving parts of Germany to Slavic nations.