While Congress did not have the power to end the international slave trade, it did have the power to regulate it, and starting in 1794, it did just that.
In March, Congress prohibited the use of any U.S. port or shipyard for the purpose of fitting out or building any ship to be used for the introduction of slaves. The law also prohibited ships sailing from U.S. ports from trafficking in foreign countries. Ships sailing from the United States to Africa, even if of foreign registry, were required to "give bond with sufficient sureties, to the treasurer of the United States, that none of the natives of Africa, or any other foreign country or place, shall be taken on board... to be transported, or sold as slaves in any other foreign place, within nine months thereafter." Penalties under the law included fines ranging from $2,000 for outfitting a ship to $200 for an individual working on such a vessel. The act provided that the ships could be confiscated, and half of all fines given to any informants, thus providing an incentive for ship captains and mariners to monitor the activities of anyone they suspected of being involved in the illegal slave trade.
Until 1800 none of the states had reopened the African trade, which had been effectively closed since the Revolution. Before 1800 all introductions into the U.S. were thus illegal, even if the slaves were brought in by foreign ships. After 1800, however, Georgia and South Carolina reopened their international slave trade, and in the next eight years, these two states would introduce about 100,000 new slaves from Africa.
With the trade legally reopened in the Deep South, Congress sought to strengthen the prohibitions on American participation in it. In 1800 Congress amended the 1794 act by dramatically increasing fines for illegal American participation and by giving informants a right to the entire value of any ship condemned under the law. In addition to not allowing American ships to participate in the trade, the new law prohibited any American from having any interest in a ship involved in the trade. Thus, Americans could no longer invest in the transatlantic slave trade, even if carried on legally by non-U.S. ships.
If convicted, an American was subject to a fine that was double the value of his investment in the vessel and also double the value of any slaves in whom he had an interest. The 1800 amendment explicitly prohibited any American citizen or resident alien from voluntarily serving "on board any foreign ship or vessel . . . employed in the slave trade." It no longer mattered if the ship was U.S., or even if it left an American port. American sailors found on slavers were now subject to a $2,000 fine. The law authorized all "commissioned vessels of the United States, to seize and take any vessel employed" in the trade contrary to the law, with the crew receiving half the value of the ship when it was sold. This provided an enormous incentive for American ships to police the trade.
With the trade legal in some states and illegal in others, in 1803 Congress provided new fines for people who brought newly imported slaves into states that banned the international slave trade. The law applied to any "negro, mulatto, or other person of color" introduced as a slave, whether from Africa or the Caribbean. The language was apparently used to prevent people who might bring in Africans by claiming they were not slaves but servants or indentured servants, or claim that they were not actually African but Caribbean.
All three of these laws had been designed to limit American participation in the transatlantic slave trade, but they could not be used to stop the trade itself. Significantly, all of the laws passed before 1807 focused on ships, sailors, and investors. None of the laws had any provision for what should happen to slaves illegally brought into the United States. Indeed, while the 1794 law provided for the sale of a ship and its "tackle, furniture, apparel and other appurtenances" of a slaver, it did not mention what should happen to the slaves or any cargo on the ship. Presumably, they too would be sold for the benefit of the United States, the informant, or any other claimant under the three laws.
In his annual message to Congress in December 1806, Thomas Jefferson, who had long opposed the trade (but not slavery itself), reminded the nation that on January 1, 1808, the constitutional suspension of congressional power on this issue would finally expire. He took a moment in his address to "congratulate" his "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 interests of our country have long been eager to proscribe." He noted that any law passed by Congress could not take effect until January 1, 1808, but he urged Congress to act quickly "to prevent by timely notice expeditions which can not be completed before that day." Congress readily complied with legislation to absolutely ban all importations of slaves after January 1, 1808.