Communism

Exactly - there's nothing inherently wrong with a group of people within a country, or even an ethnic group, banding together in order to advance their common interests as a People.

Where do things go wrong? When you ask: "What are the interests of that Nation? And what do you plan to do to advance them?"

An American Nationalist must realize that it is in the best interests of the nation to maintain Imperialism and exploitation overseas, and depending on how they define their nation, exploitation of immigrants and ethnic minorities at home as well.