greatest paradox in US in 19th century?

Question

Was slavery the paradox of 19th century?

Answer 1

No. It was the paradox of the 18th century, when Enlightenment minds were declaring equality. By the 19th century, it had been built into the paradigm.
The 19th century paradox was the willingness to import international issues (Louisiana Purchase, California Gold Rush) without agreeing to export American values.

Answer 2

I like Mirabell's answers. I would add that the 19th century was when we started a war with Mexico as part of our expansionist philosophy--and we got what is now Arizona, New Mexico, Utah and Colorado. At the end of the century, we were embroiled in the Spanish-American War which yielded us control over Cuba, Puerto Rico, the Philippines and Guam. (We had already gained control of California and Texas).

Doesn't quite fit with our sense of fairness and democracy, but it is part of our history.