German nationalism is the idea that the different people who are seen as German belong together and should make one country. German nationalism started around 1770. At the start, there were many different small kingdoms and duchies, the German state only started to exist in the mid-19th century. German nationalism started with the Napoleonic wars, when France invaded the different German states. In the 19th century, the main question was whether to make a Germany that included the modern-day state of Austria, (seen as lesser German) or to make one that included it.
In the end Bismarck opted for a lesser German solution (which did not include Austria). This is through German Unification. When the Nazis came to power in 1933, they wanted to create a Greater Germany (including Austria). During this time, the people who were not seen as German were seen as Untermenschen (subhumans), and many were killed.