The United States of America

________ was legal in the southern United States until the second half of the 19th century, when the American Civil War led to its abolition.

________ migrated from Siberia to the North American mainland at least 12, 000 years ago, and European colonization began in the 16th century.

What is Washington, D.C., and the most populous city is New York City? Answer with a single words or term.