So the effort to abolish the "right" of a slaver to starve, suffocate, mutilate, torture, or murder a black man in whom he had acquired a property right by the simple process of kidnapping required more than half a century to attain complete success.
The first serious blow to the slave-trade fell in 1772, when an English court declared that any slave coming into England straightway became free.
That closed all English ports to the slavers.

Two years after the American colonists, then on the threshold of the revolt against Great Britain, thought to put America on a like high plane, and formally resolved that they would "not purchase any slave imported after the first day of December next; after which time, we will wholly discontinue the slave-trade, and will neither be concerned in it ourselves, nor will we hire our vessels, nor sell our commodities or manufactures to those who are concerned in it." But to this praiseworthy determination the colonists were unable to live up, and in 1776, when Jefferson proposed to put into the Declaration of Independence the charge that the British King had forced the slave-trade on the colonies, a proper sense of their own guilt made the delegates oppose it.
It was in England that the first earnest effort to break up the slave-trade began.

It was under the Stars and Stripes that the slavers longest protected their murderous traffic.

For a time the effort of the British humanitarians was confined to the amelioration of the conditions of the trade, prescribing space to be given each slave, prescribing surgeons, and offering bounties to be paid captains who lost less than two per cent.

of their cargoes on the voyage.