Final answer:
The decade in which the importation of enslaved people into the United States was made illegal is 1800-1810, specifically after a law was passed in 1807 and went into effect on January 1, 1808.
Step-by-step explanation:
The decade during which the importation of enslaved people was made illegal in the United States is 1800-1810. Specifically, in 1807, the U.S. Congress passed a law that prohibited the importation of slaves into the country, and this law came into effect on January 1, 1808. Despite the federal ban, the domestic slave trade flourished as demand for labor within the United States surged, especially with the cultivation of cotton.
States like Virginia and Maryland became leading participants in the internal trade, selling 'surplus' enslaved people to agriculture in other Southern states. This transfer involved one of the largest forced internal migrations in U.S. history, disrupting innumerable lives and separating families.