It was similar in some ways to the unification conflicts happening in Europe in the 19th century. The contentious issues, slavery and tariffs, forced a disagreement about the nature of the Union. There was not a shared sense of identity across the United States, so when Virginia, for example, seceded, Virginians sided with their state. Generally, only the more cosmopolitan upper classes felt conflict between a duty to preserve the union and a state citizenship; the enlisted ranks and home support were more uniformly provincial. In the north, an almost religious belief in the moral and social good of a nationalist Union drove pro-war sentiment (again, similar to other unifications.)

It’s an expansive topic; I’d encourage you to pick a few accounts of the era as well as biographies of Lincoln and generals Grant and Lee.