The moral case against slavery evolved gradually. Its origins were religious. The Christian teaching that all human beings are equal in the sight of God was not of course a political statement. But it established a baseline concept of equality with the potential to undermine the rigidly hierarchical organization of human society that had existed throughout recorded history.
In Christian Europe the Church was seldom to be found on the side of the poor and oppressed. Notoriously, Martin Luther exhorted the German princes to crush the peasant rebellion, calling it an abomination against the divinely sanctioned order of society. But the Protestant Reformation, by undermining religious authority, undermined traditional political authority as well. If religious congregations were competent to govern themselves in the religious sphere, why not in the political sphere also? If bishops were to be deposed, why not kings and princes also? These ideas were a driving force of the English Civil War, which deposed divine right in favor of government by consent.
Each step along this political path undermined the institution of slavery, which in the European context was largely a consequence of colonial expansion. The need for plantation labor on the sugar islands and mainlands of the New World gave rise to the African slave trade. Slaves meant profits; but very obviously slavery was incompatible with evolving doctrines of political rights and government by consent. The men who wrote the Constitution of the United States were well aware of this. They knew that union was only possible if slavery in the southern states was tolerated for the time being. But they knew also that slavery was incompatible with the principles they were espousing, hence the Constitution’s reticence on the subject. The word “slavery” never appears.
The English, American and French revolutions set the stage for the nineteenth-century Abolition movement. Now the doctrine of what we would call human rights had been openly proclaimed and was being steadily developed. Abolition went hand in hand with the steady expansion of civil rights in Britain and America. And, appropriately enough considering the origins of the moral case against slavery, Abolition acquired a strong tincture of religion.
Of course racism persisted; even many Abolitionists doubted that blacks were or could ever be equal to whites. But that blacks, as human beings, possessed baseline natural rights now seemed undeniable, and to accept this proposition was to reject a social order whereby one man could be another man’s property. The case against slavery was further buttressed by the exposure of its horrors, e.g. in Uncle Tom’s Cabin and slave narratives like those of Frederick Douglass. The extent to which these revelations shocked the Victorian conscience ought not to be underestimated.
That Abolition coincided with the rise of European and American imperialism is an irony of history and it invites a charge of hypocrisy. But history is seldom so neat and tidy as to preclude the temptations of cynicism. Anyhow, the same forces that destroyed chattel slavery in the nineteenth century undermined the foundations of imperialism in the twentieth century.