Nationalism is an ideology that fosters deep attachment to one's nation, whereas sectionalism is allegiance to one's particular section of the country. The distinction between nationalism and sectionalism is most significant in describing the political climate of the antebellum era of the United States, when sectional conflict between the north and south overwhelmed nationalist feelings and led the country to civil war.
The Stanford Encyclopedia of Philosophy explains that nationalism is a spirit of intense affinity for one's nation, which is a ethnic and cultural unit. Nationalism emphasizes the value of the traditions, practices, customs, institutions, language and history of the nation, often romanticizing these elements and downplaying imperfections. In the context of history, nationalism is often used to characterize the zeal that provoked the nations of Europe to engage in the two world wars.
Sectionalism of the degree seen in Civil War era America is rare among most nation-states because the nation is typically a single cultural entity. Thus, smaller political units, like cities, do not have enough cultural independence to inspire an allegiance stronger than one's loyalty to the nation. Because the United States was a union of autonomous states with their own identity, the American people prior to the civil war did not have a strong national identity and tended to place allegiance to their own state above allegiance to the country.