White men with rifles.
In Black Rednecks and White Liberals, Thomas Sowell points out that slavery existed as an accepted institution unchallenged throughout history, worldwide, until the Eighteenth Century. Throughout that long era Christians did not reject the institution (for other than themselves and their kin) any more than the Hindus or the pagans or the Buddhists or the Confuscians or the Muslims did.
No literature existed anywhere to defend the institution of slavery before the such literature started to appear in the American south, because until then the institution of slavery had never been under consistent and determined attack. Christians didn't begin to mount effective opposition to the institution of slavery until the Eighteenth Century but no other cultural ifluence ever has mounted such opposition.
The British underwent the expense of maintaining a naval squadron off the west coast of Africa for no other purpose than suppressing the slave trade which - apart from the moral issue - was absolutely no skin off Britain's nose. Indeed, the purpose of the Emancipation Proclamation - which "freed" only those slaves where its writ did not run - was to take the possibility of British recognition of the Confederacy off the table. The British industrial revolution was centered in its fabric mills, and those mills ran on cotton; the British economic interest alone would have dictated British recognition of the Confederacy, which could have doomed Union efforts to suppress the Confederacy. But with the Lincoln Administration having officially stipulated that the Civil WAr was not only about defending the Union but about abolishing slavery, British opposition to slavery precluded British support for the Confederacy.
The (huge) British Empire continued to exert pressure against slavery, inducing people who had no interest in its abolition to assent for the sake of relations with Britain.
I didn't like this book: the first part was fanstastic, but a couple of the other essays were less than thrilling.