Historian Eric Foner states: Slavery was intimately related to the major trends [and] developments that we associate with American history in the first half of the 19th century. For example, territorial expansion, the westward movement, the frontier. The country grew tremendously in this period until, by the 1840's, it reached the Pacific Ocean. Frederick Jackson Turner, the great historian of the late 19th century, said it was on the frontier that democracy was born, that American ideas of equality were born, individualism.
But the frontier also carried with it the expansion of slavery. The westward expansion of slavery was one of the most dynamic economic and social processes going on in this country. The westward expansion carried slavery down into the Southwest, into Mississippi, Alabama, crossing the Mississippi River into Louisiana. Finally, by the 1840's, it was pouring into Texas. So the expansion of slavery, which became the major political question of the 1850's, was not just a political issue. It was a fact of life that every American had experienced during this period.
Americans in the 19th century thought of or spoke of their country as in Jefferson phrase -- an "empire of liberty." And the history of the United States was conceived of as part of the progress of mankind and the spread of liberty throughout the world. And you can see this in graphic illustrations of the period -- of liberty leading people westward. And progress was the essence of the American story.
Now, in the South, southern slaveowners insisted that slavery was absolutely essential to that story of progress. Without slavery, you could not have civilization, they said. Slavery freed the upper class from the need to do manual labor, to worry about economic day-to-day realities, and therefore gave them the time and the intellectual ability to devote themselves to the arts and literature and mechanical advantages and inventions of all kinds. So that it was slavery itself which made the progress of civilization possible.
Now, northerners by this period wouldn't have put it exactly that way, because they lived in a non-slave area. But I think in the North, the connection of slavery and American growth was really sort of ignored. In other words, people would talk about the expansion of the "empire of liberty" and never quite mention that millions of people in this "empire of liberty" were slaves.