How did nationalism impact large empires during the 19th century?

It was in the 19th cent. that nationalism became a widespread and powerful o achieved national unification and freedom from foreign domination but certain areas inhabited by Italians were not included in the new state and this gave rise to the problem of irredentism. In the United States, where nationalism had evinced itself in the doctrine of Manifest Destiny national unity was maintained at the cost of the Civil War.