The war's root cause was the fight over slavery's expansion, not slavery's mere existence

McPherson emphasizes that the immediate political flashpoint driving the country toward war was not simply the existence of slavery in the South, which most Northern politicians were willing to tolerate, but the fiercely contested question of whether slavery would be permitted to expand into the vast new western territories acquired through war and purchase. Each new territory or state application for admission reopened this fight, since it would tip the delicate balance of political power between free and slave states in Congress. Compromises like the Missouri Compromise and later the Compromise of 1850 were attempts to manage this expansion question through geographic lines and popular sovereignty formulas, but each proved temporary as new territory and new circumstances reopened the conflict. McPherson shows that this framing explains why seemingly technical legislative fights, like the Kansas-Nebraska Act, provoked disproportionate outrage and violence, since they were understood by contemporaries as determining the future balance of slave versus free political power for the entire nation, not just a regional or local matter.

Takeaway: the crisis wasn't slavery's existence alone but the unresolved question of where its expansion would stop.