When the Constitution was drafted Congress was granted the power to ban the international slave trade into the US, but the power could not be exercised until January 1, 1808. President Thomas Jefferson in his annual message to Congress of December 2, 1806, noted that January 1, 1808 was approaching:
I congratulate you, fellow citizens, on the approach of the period at which you may interpose your authority constitutionally to withdraw the citizens of the United States from all further participation in those violations of human rights which have been so long continued on the unoffending inhabitants of Africa, and which the morality, the reputation, and the best of our country have long been eager to proscribe. Although no law you may pass can take prohibitory effect Ôtil the 1st day of the year 1808, yet the intervening period is not too long to prevent by timely notice expeditions which can not be completed before that day.
Congress in 1794 had passed an act to prohibit American ships from participating in the slave trade, so ships bringing slaves into the US could do so legally only if they were foreign ships. Congress acted promptly in 1807 and a law banning the importation of slaves was signed by Jefferson on March 2, 1807. Considering the furor that surrounded slavery in the coming decades, it should be noted that the banning of the international slave trade was not controversial and enjoyed broad support in Congress.