DURING THE 1840S AND 1850S, a cluster of interrelated developments — population growth, steam power, railroads, and the growing mechanization of agriculture and manufacturing — meant greater economic productivity, a burst of output from farms and factories, and prosperity for many. Diplomacy with Great Britain and war with Mexico handed the United States 1.2 million square miles and more than 1,000 miles of Pacific coastline. One prize of manifest destiny, California, almost immediately rewarded its new owners with tons of gold. Most Americans believed that the new territory and vast riches were appropriate rewards for the nation’s stunning economic progress and superior institutions.

To Northerners, industrial evolution confirmed the choice they had made to eliminate slavery and promote free labor as the key to independence, equality, and prosperity. Like Abraham Lincoln, millions of Americans could point to their personal experiences as evidence of the practical truth of the free-labor ideal. But millions of others knew that in the free-labor system, poverty and wealth continued to rub shoulders. Free-labor enthusiasts denied that the problems were inherent in the country’s social and economic systems. Instead, they argued, most social ills — including poverty and dependency — sprang from individual deficiencies. Consequently, many reformers focused on personal self-control and discipline, on avoiding sin and alcohol. Other reformers focused on woman’s rights and the abolition of slavery. They challenged widespread conceptions of male supremacy and black inferiority, but neither group managed to overcome the prevailing free-labor ideology based on individualism, racial prejudice, and notions of male superiority.

By midcentury, half of the nation had prohibited slavery, and half permitted it. The North and the South were animated by different economic interests, cultural values, and political aims. Each celebrated its regional identity and increasingly disparaged that of the other. Not even the victory over Mexico could bridge the deepening divide between North and South.