1850s: The United States is an increasingly divided country. Tensions flare between Southerners and Northerners, and two presidents fail to ease the conflict over slavery and ideology that is building steadily toward civil war.

Today: The United States appears to be a divided country once again. Republicans and Democrats have deep ideological differences, and the administration of President George W. Bush is known for rewarding its ultraconservative base and refusing to take a moderate stance.

1850s: Boston is the literary and intellectual hub of the United States, boasting the greatest thinkers and scholars of the American Renaissance.

Today: Although Boston remains a center of American intellectual life, home to many of the best universities in the country, New York is a larger hub of literary and philosophical thought.

1850s: In the United States, slavery is legal in Southern states, African Americans throughout the country...