The experience of being Thomas Jefferson’s biographer was a salutary one for me (perhaps especially because I have acquired an unjustified reputation for being more willing to attack people than to praise them).

America’s third president was a true man of the Enlightenment, making original contributions in every field from vaccination against smallpox to the cataloguing of Native American languages. During his time in office, he doubled the size of the country by shrewdly making the Louisiana Purchase, thus more or less putting an end to the domination of North America by British, French and Spanish imperialism. He sent the American fleet to the Mediterranean, to put down the slave trade operated by the piratical Islamic regimes of North Africa, and to protect free trade and free navigation. But he was also a pioneer in the use of sanctions as an alternative to war, and in the proposing of an international “concert of nations” to keep the peace.

Nor were his achievements solely presidential. As a Virginia politician, he drafted the world’s first statute for the separation of church and state, which became the basis of the First Amendment to the Constitution. All supporters of secularism against superstition owe him an enormous debt: his voluminous writings on the subject have been a permanent safeguard against the regular sectarian attempts to “Christianise” the United States.

I had, of course, understood that, like most of the American founding fathers, he held human beings as private property. Unlike most of them, he opposed slavery on principle, but could never find the right political occasion to put his principle into practice. (That is to say, he was able to make progress against the trade in slaves, but not against the institution of slavery on American soil.) It also seemed probable that he had fathered several children with one of his chattels: a girl named Sally Hemmings who was (by one of those grotesque features of plantation life) fathered by his father-in-law.

Taking these things in reverse order, I would now say that the affair with Sally was less repulsive than it looks: there is good evidence that it was her idea, that it was more like a secret marriage than mere owner-slave exploitation, and that it led to the freeing of all their children. However, it becomes clear from reading Jefferson’s antislavery writings that he was in favor of manumission, but not emancipation: in other words the slaves should be freed but they should not be allowed to stay on American soil. They were to be deported either back to Africa or to colonies in the Caribbean. (Jefferson never lost his fear of slave revenge, and was horrified when a slave rebellion broke out in Haiti under the leadership of Tous-saint L’Ouverture. The irony here is that it was the Haitian revolution that destroyed Bonaparte’s Caribbean army and navy, and forced him to sell New Orleans and then all of Louisiana, so Jefferson was taking credit for the consequences of a movement that he viciously opposed.)

It was actually Thomas Paine who helped suggest the purchase of Louisiana, and it was Paine who pointed out that it gave America an opportunity to cancel its original sin. Slavery should be banned in the new territories, he beseeched his friend in the White House, and freed slaves should be given grants of land there. He, and other radicals such as Joel Barlow, were profoundly shocked and betrayed when Jefferson opened the new land to slavery, capitulating to the sugar interest that needed a labour-intensive underclass in the same way as he had upheld the tobacco economy in his native Virginia. The moral and ethical questions to one side, the admission of new slave states to the Union could only have one consequence — that of an eventual civil war. It is difficult to avoid the conclusion that it was Jefferson who bequeathed this awful future to the country he had helped to create.

American Marxists used to cite Jefferson as their favourite founder, and the Jefferson School in New York was a hotbed for the left. The late IF Stone used to describe himself as “a Jeffersonian Marxist”, only to find that he got into more trouble for claiming the first allegiance than he did for the second one. When asked how he reconciled himself to the slavery question, he would reply “because history is a tragedy, and not a morality tale”. I annexed this thought for the closing sentence of my book. However, I did learn that there is one excuse a historian or biographer must never make. It will never do to say that “standards were different in those days”. Many of Jefferson’s contemporaries understood quite well, as he did, that slavery was both a crime and a threat to the survival of the Union. The element of tragedy arises precisely from the way in which historical figures know that what they are doing is fatal, but continue to do it anyway.

Christopher Hitchens is a columnist for Vanity Fair. His latest biography, Thomas Jefferson: Author of America, is published by HarperCollins

Never less than provocative

One of the highlights of this year’s Sunday Times Oxford Literary Festival will be Christopher Hitchens’s encounter with Nick Cohen, author of What’s Left? How the Liberals Lost Their Way. Cohen’s book, about the divided response of the left to Islamism and the war on terror, has already provoked heated debate, and was praised by Hitchens in these pages as “an admirable example of self-criticism and self-examination, using intellectual honesty as a means of illuminating a much wider canvas”. They will discuss issues thrown up by the book, and by Hitchens’s biography of Thomas Jefferson, on Sunday, March 25 at 2pm.

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