The record's pretty mixed. The US takes vigorous action to oppose the slave trade, particular imports to the US, but then loses interest. The policies it does adopt for suppression are often enforced intermittently and otherwise ignored. But it also becomes the leading maritime power protecting and facilitating the transatlantic trade.

There are two slave trades of concern here. The first is the kind outlawed in 1807, effective 1 January 1808. The importation of slaves to the United States does not cease instantly, but it does decline considerably. We used to think otherwise, with estimates as high as 250,000 illegally imported slaves, and we still can't get firm numbers because of the usual issues with looking at illicit behavior, but it's more likely that direct imports from Africa (which are a little easier to find than others I'll get to shortly) are about 1,000 a year between 1810 ad 1820. That's down from tens of thousands pre-ban. Things decline further to about 200 per year in the 1820s, then nearly zero afterwards. The total is around 12,000 people between 1810 and 1860.

But that's only part of the importation story. For much of this time the United States had land borders with other enslaving powers: Spanish Florida and Texas. (There's also probably some that come via ship from Cuba.) These borders were far from anywhere and not very policed at all. They were also immediately adjacent to slave-hungry regions of the United States, Georgia and Louisiana in particular. Imports to Louisiana in particular aren't so much ignored or shrugged off as enthusiastically welcomed by locals. The volume is big enough that the anti-importation laws are strengthened (mostly) and eventually slave trading is classed as piracy, punishable by death. (Also the contraband slaves are now to be sent back to Africa and funds were appropriated for the expense.)

None of this translates to unified policy on the ground. Enforcement is left to US district attorneys, judges, juries, territorial governors, and the US Navy with the cooperation of states and private individuals. Nobody is really in charge of this, except the president in a distant sense. There's not much coordination or chain of command involved on the government side. So you get anything from really gung-ho types trying to get the Navy to come and break up operations to judges who hand out slaps on the wrist to guilty parties to federal officials who allowed the import of a hundred people, had charges recommended against them, and retained their offices.

That said, some of the bigger operations do get busted up. In 1817 the Navy and Army move against Amelia Island, at the mouth of St. Mary's river in Spanish Florida. It's working basically as a slave importing depot, with slaves marched from there by land or carried by water to Georgia. On the other side of the country, Jean Laffitte is a big player in an operation from Galveston Island. He and others like him would seize slaves from the Spanish and take them in for sale, about a dollar a pound. Importers would come down and buy them, then march them overland to Louisiana. Jim Bowie was involved in that. The Navy moves on them in 1821. Those two actions, combined with increased stability and consequent lack of opportunity as the Latin American revolutions wind down and the acquisition of Florida, cut off much of the flow of illicit slaves into the United States.

Those actions are all basically around the margins of the US, albeit with Galveston being a rather longer haul than Amelia Island. Until 1819, that's about all the US does in the way of suppression. Starting that year, there's a naval squadron detailed to cruise off the African coast. It's supposed to be a regular presence and does stop some slavers engaged in the foreign slave trade to supply Cuba and Brazil, mostly. The African Squadron's general performance is a bit of a mess, though. It starts well, then falls off. Coordination with British efforts is often absent. Sometimes, so is the whole squadron. There are years when it's just a perfunctory call at Liberia and others where a lot of time is spent in ports just sort of hanging out. The Navy has many other priorities. It's natural to suspect a sectional bias here, with southerners shirking and Yankees being diligent, but the sections perform about equally in the water.

As the USN slacks off, the Atlantic slave trade enters what's likely its highest-volume years. Most of that trade is to foreign ports and Americans are not hugely involved until 1835. That's when things turn from ineffective to pretty ugly. From the 1820s to the 1850s, slavers become increasingly American. Baltimore is a major center of slave ship production and Americans get more personally involved to go along with their financial stake.

American policy is, on paper, pretty nasty. The British wouldn't execute you for slaving; the US would. But the US provides the flag of convenience for the outgoing leg of the voyage because American law is largely concerned with the act itself and not so much the precursors. You don't technically need to be caught with slaves in the hold, but it tends to work out to that. The language about fitting out and having equipment on board for slaving voyages is left undetailed in American law, where the UK develops a much more specific system that makes its provisions enforceable.

This causes frequent foreign policy irritation as Britain steps up its effort to suppress the trade. It's a standard of maritime law that outside territorial waters, your navy can only freely stop and board your own ships. Through a combination of diplomatic pressure and cash, the UK convinces Atlantic maritime powers to assent to treaties permitting mutual searches and seizures of suspected slavers. The idea was that the Royal Navy could serve as the seafaring slavery police, with seized ships taken before commissions with members from both nations concerned to be libeled, condemned, and sold. The Spanish, a major slaving power, sign on in 1817 with the help of a large bribe. The Dutch do so the next year, sans bribe.

On the American side, the response to the treaty system is nearly pretty hostile. There are doubts about whether or not the Constitution allows the US to consent to having Americans tried by a foreign power. It would be an entanglement with a European power and Americans tend toward exceptional wariness on that front, doubly so when the power is the UK. And then there's the mutual search itself, which looks akin to impressment and they just finished the War of 1812. The rest could be massaged away, but not this offense on freedom of the seas. At one point, his British counterpart asks John Quincy Adams if he can imagine an evil worse than the slave trade. Adams, who has more sterling antislavery credentials than just about anybody of prominence in the era, answered back:

Yes, admitting the right of search by foreign officers of our vessels upon the seas in time of peace; for that would be making slaves of ourselves.

There is some cooperation on designating slavery as piracy, which the British agree to on Adams' suggestion, but mutual search is just not happening. So if you see a Royal Navy vessel approaching and you have a full of chains, you run up Old Glory. The more the British stepped up their efforts to convince other nations to join in suppression, the more the American flag became preferred. Louis Phillippe's France signs on with Britain for mutual search and shortly thereafter the Portuguese are strongarmed into compliance. But the banner event pushing the foreign slave trade to Americans is probably when Spain agreed to let the British seize ships that were outfitted for slaving, even with no slaves on board. That puts the Cuban trade, which is right there over into American-flagged ships.

Americans built many of those ships and facilitated fake sales that preserved American nationality for them while allowing Spanish ownership. So a slaver from Cuba could basically buy a ship, outfit it, and send it to Africa under the American flag. Between refusal by the US to accept British inspection and American non-enforcement, that's an outgoing trip conducted with impunity. Of course once you take people on, you run up the Spanish flag. That subjects you to British capture, but the British are only going to seize the ship and human cargo. The captain and crew will walk, where the US would possibly fine and/or execute.

This could get pretty flagrant. For some time, the consul in Havana was Nicholas Trist. (The same Trist who negotiates the Treaty of Guadalupe-Hidalgo.) Trist would sign almost any papers someone put in front of him granting the protection of the US flag and American policy to slavers. He was such a go-getter that when the Portuguese consul was gone for a year plus, he probably did the same for them.

(back in a moment)

Wasn't the British Navy involved in operations again the slave trade even prior to the US Navy? Were they more or less effective? Did they operate in different areas? Did they interrupt Brazilian slave trade or only North American and Caribbean?

On March 2, 1807, the 9th Congress prohibited the importation of slaves. Illegal trades began to come from the British Caribbean after May in 1807 and continued through the various Carribean nations. It is estimated that about 1.5 million slaves, or about 15% of the those who survived the middle passage, were imported into the Americas illegally. The slave trade did not show much gradual decline as the need for slave labor proved to be greater as the need for production increased.

As to United States Navy ships disrupting transatlantic slave traders, the United States Navy and its British counterpart did band together to intercept slave ships, one ship that comes to mind is the Wanderer. The United States Navy formed the African Slave Trade Patrol in 1819 but the Patrol was largely ineffective. In my opinion, the efforts by the United States Navy were largely ineffective.

The Royal Navy was more successful than its American counterpart when it came to suppressing the illegal slave trade. The Preventative Squadron, also called the West Africa Squadron, was able to capture sixteen times more ships than the African Slave Trade Patrol. Europe was much more effective in deterring slave trading after the convention of the Congress of Vienna in 1814.