Answer:
Because some slaves were purchased illegally.
Step-by-step explanation:
At the time of the American Revolution (1775–83), there was widespread support in the northern American colonies for prohibiting the importation of more slaves. However, after the revolution, at the insistence of Southern states, Congress waited more than two decades before making the importation of slaves illegal. When Congress did so, in 1808, the law was enacted with little dissent, but Caribbean smugglers frequently violated the law until it was enforced by the Northern blockade of the South in 1861 during the American Civil War.
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