Sectionalism, or loyalty to one's region or section of a country, has been an important part of American history since the very founding of the United States. The original colonists' primary attachment was to their individual states, and each had its own unique identity. To a large extent, this was a reaction against the centralized nature of British colonial rule and found outward expression in the loosely-structured government established by the Articles of Confederation.
Sectionalism was given added force by the vast divergence in economic development between North and South. The Northern economy was more industrialized, with a substantial role for banking and commerce. The Southern economy, by contrast, was overwhelmingly agricultural, and based on slave labor. And it was the thorny issue of slavery that brought sectional tensions to the boil. Despite numerous attempts at compromise, it became clear that only some kind of radical constitutional upheaval or armed conflict would be able to break the impasse. Although the ensuing Civil War didn't entirely remove sectionalism as a force from American political life, it was considerably marginalized, nonetheless. As the great American historian Shelby Foote once said,
Before the war, it was said "the United States are"—grammatically it was spoken that way and thought of as a collection of independent states. And after the war it was always "the United States is," as we say today without being self-conscious at all. And that sums up what the war accomplished. It made us an "is."