For 50 years, England dominated the worldwide slave trade, transporting over half of all African slaves, making an enormous profit. In 1811 England made the slave trade illegal worldwide, and her ships suddenly shifted roles from transporting slaves to searching for ships transporting slaves, and punishing violators.
On what authority was England now the world's slave-trade policeman? Apparently, her power, wealth, and vast navy provided all the international legal authority required. For all, that is, except the United States. American ships refused to allow the Brits to stop and inspect them on the seas, because the New England states were still making a very handsome profit in the slave trade, and they had no intention of giving that up any time soon. (In spite of the fact that it was illegal in the US, also.) So other countries made a point of having at least one American on board each slave-trade vessel, and if approached by a British ship, the American flag would be hoisted temporarily, or the ship would be very quickly sold to the American on board.
Note that it was New England ships that were engaged in the slave trade, not Southern ships. The South embraced laws against the slave trade long before New England finally was brought into compliance, about the time of the Civil War. Note also that it was the American flag that flew on those vessels. The Confederate flag never flew on a slave ship, and the Confederate Constitution specifically prohibited the slave trade.
Even though England may have overstepped her rightful international legal authority in assuming the role as the world's slave-trade policeman, it is still encouraging to see that she had finally come to her senses and tried to make amends for all those years of slave-trade profiteering. Finally, compassion triumphed over greed.
Well, not really. You see, Haiti was quite a thorn in Britain's side, because that little French island was the chief competitor for England in the Caribbean area. Since Haiti's economy was totally dependent on slave labor, England hoped to put Haiti out of business along with slave-traders. Still, Britain gained the reputation as champions of the world's most oppressed people. Which shows that rewriting history is one of the world's favorite and most entertaining sports.
Learn more about the American Civil War here.