Great Britain and specifically the British Royal Navy were critical elements in reducing global slavery. In 1807 the British passed the Slave Trade Act which outlawed the slave trade within the very large British Empire. To help enforce this act, the British Royal Navy was given the power to consider slave ships as pirates. If they found any at sea, they would board them and free any slaves that were captured. Between 1808 and 1860 they freed around 150,000 would be slaves from more than 1500 captured ships.
English map from 1850. Not the detail on the west coast of Africa, while the interior is comparatively incomplete.
A Royal Navy squadron that was known as the West African Squadron was tasked with stopping the slave trade off of the African coast. British captains would earn a bounty for captured slavers. This is an example of the types of orders given to captains. It outlines what treaties must be followed.
The size of this squadron started small, but grew in size to be about a full sixth of the entire navy. In 1842, the United States formally started helping with the effort with American ships.
Britain also used its strength to pressure other countries to outlaw the slave trade. Between 1808 and 1820 Britain signed treaties to outlaw the slave trade with Portugal, Sweden, France and the Netherlands. They also signed a treaty with the United States to end importation of slaves, though internal slavery was still allowed with the United States until the Civil War ended it.
The following chart from the Slave Voyage database shows how the various treaties reduced the flow of slaves.
This social media post argues that the progress against slavery went through fits and starts but made “substantial progress”.
Slavery had not been abolished outside the British empire. Anti-slavery societies, the British government, the Royal Navy, enforcing anti-slavery conventions, and the governments of other Western powers continued to work for general abolition into the second half of the nineteenth century. Cases of British subjects in slavery continued to cause widespread outrage, a litmus test of the commitment of government to abolition of slavery wherever it occurred. A guide for naval officers set out for them the legal framework that was created for abolition, listing some twenty-seven groups of treaties, conventions, engagements and declarations from 1817 to 1842, with European and American states, and African kingdoms and chiefdoms. Putting this into operation was complex. By about 1865, however, very substantial progress had been made; the trade to South America was largely stopped.
The flow of slaves from the west coast of Africa slowed dramatically after the American Civil War. After that time, Great Britain started to focus their anti-slavery efforts on the east coast of Africa and the Arabian slave trade. The efforts lasted until the early part of the 20th century.
Other groups, especially private abolitionist groups were very important to ending slavery, but the British Navy played a critical role in reducing global slavery.