Sectionalism is similar to nationalism but they differ in the fact that Nationalism is the pride in one's country while sectionalism is the pride of one's region, state, or section. For example, Americans took pride in being a "New Englander" a "southerner" a "Virginian" etc. Sectionalism can be described as the action that happens when an American identifies himself more with a region or section, than more than the whole nation.
In the early times, Sectionalism was big but after wars and battles won, Americans directed their pride to the nation. After that, Sectionalism occurred and people started to have more love for their own regions rather than a whole country.