Sectionalism is the idea that a person is loyal to the region in which they live rather than their country. This was seen in the United States, especially during the pre-Civil War years. During this time, also known as the antebellum era, many Northerners and Southerners identified themselves by the region they live rather than as Americans. This was caused by several different factors, one of the most prevelant being whether or not the institution of slavery should be expanded into new territories gained by the US during westward expansion.
Sectionalism in an exaggerated attention to one's own section, or group or local interests. Another way of saying it is that it is a way to divide countries, states and cities.
