The importation of African slaves was outlawed by an act of Congress passed in 1807, and signed into law by President Thomas Jefferson. The law was rooted in an obscure passage in the U.S. Constitution, which had stipulated that importing slaves could be prohibited 25 years after the ratification of the Constitution.
Though the end of the international slave trade was a significant piece of legislation, it actually did not change much in a practical sense. The importation of slaves had already been decreasing since the late 1700s. However, had the law had not gone into effect, the importation of slaves many have accelerated as the growth of the cotton industry accelerated following the widespread adoption of the cotton gin.
It's important to note that the prohibition against importing African slaves did nothing to control the domestic traffic in slaves and the interstate slave trade. In some states, such as Virginia, changes in farming and the economy meant slave owners did not need great numbers of slaves.
Meanwhile, planters of cotton and sugar in the Deep South needed a steady supply of new slaves. So a thriving slave-trading business developed in which slaves would typically sent southward. It was common for slaves to be shipped from Virginia ports to New Orleans, for instance. Solomon Northup, the author of the memoir Twelve Years a Slave, endured being sent from Virginia to bondage on Louisiana plantations.
And, of course, an illegal traffic in slave trading across the Atlantic Ocean still continued. Ships of the U.S. Navy, sailing in what was called the African Squadron, were eventually dispatched to defeat the illegal trade.
The 1807 Ban on Importing Slaves
When the US Constitution was written in 1787, a generally overlooked and peculiar provision was included in Article I, the part of the document dealing with the duties of the legislative branch:
Section 9. The migration or importation of such persons as any of the states now existing shall think proper to admit, shall not be prohibited by the Congress prior to the year one thousand eight hundred and eight, but a tax or duty may be imposed on such importation, not exceeding ten dollars for each person.
In other words, the government could not ban the importation of slaves for 20 years after the adoption of the Constitution. And as the designated year 1808 approached, those opposed to slavery began making plans for legislation that would outlaw the trans-Atlantic slave trade.
A senator from Vermont first introduced a bill to ban the importation of slaves in late 1805, and President Thomas Jefferson recommended the same course of action in his annual address to Congress a year later, in December 1806.
The law was finally passed by both houses of Congress on March 2, 1807, and Jefferson signed it into law on March 3, 1807. However, given the restriction imposed by Article I, Section 9 of the Constitution, the law would only become effective on January 1, 1808.
The law had 10 sections. The first section specifically outlawed the importation of slaves:
"Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled,That from and after the first day of January, one thousand eight hundred and eight, it shall not be lawful to import or bring into the United States or the territories thereof from any foreign kingdom, place, or country, any negro, mulatto, or person of colour, with intent to hold, sell, or dispose of such negro, mulatto, or person of colour, as a slave, or to be held to service or labour."
The following sections set penalties for violations of the law, specified that it would be illegal to fit out ships in American waters to transport slaves, and stated that the U.S. Navy would enforce the law on the high seas.
In subsequent years the law was often enforced by the Navy, which dispatched vessels to seize suspected slave ships. The African Squadron patrolled the west coast of Africa for decades, interdicting ships suspected of carrying slaves.
The 1807 law ending the importation of slaves did nothing to stop the buying and selling of slaves within the United States. And, of course, the controversy over slavery would continue for decades, and would not be finally resolved until the end of the Civil War and the passage of the 13th Amendment to the Constitution.