Sectionalism is a concern and loyalty to a distinct part of a country over the entirety of it. For example, in the history of the United States, the Southern citizens were loyal to Southern states, not the whole nation. It had ultimately led to secession and war.
Any country can be divided into regions or subcultures based on some unique characteristic that distinguishes these distinct parts from the rest of the country. Sectionalism can be described as loyalty to that part rather than the whole country. The reasons for it can be multifaceted and diverse. The most well-known examples, unfortunately, have to do with violent separatism and military action against the rest of the nation.
The American Civil War is, perhaps, the most famous example of sectionalism. In the antebellum, the Southern states were concerned with their economic stability and upholding their autonomy. The Union wanted to impose its laws upon the South, which created tension between the two sections of the country. The sectionalism, in this case, is represented by the Southern politicians and wealthy plantation owners. They wanted to uphold their right to own slaves over the right of slaves to be free. They deemed it necessary to uphold their social order and economic system, in which slave labor was used to produce cotton, tobacco, and sugar, which were then sold for profit. The rest of the country supported the industrial economy and slave liberation over the agrarian economic system based on slavery. This conflict of interest eventually led to secession and the Civil War. It would not be the end of sectionalism in the United States, however, as the South would retain its loyalty to their way of life during the Jim Crow era over the wellbeing of African American citizens and Northern meddlers. Some sectionalist and separatist attitudes persist to this day when citizens prioritize the interests of the individual states over the needs of the whole nation.