US the largest slave system?

If I remember correctly, Enrico Dal Lago's American Slavery, Atlantic Slavery, and Beyond has some helpful comments about the Sokoto Caliphate and the U.S. on this point.

When you say "the largest" do you refer to the size of enslaved population?

Yes, I'm thinking about the size of the enslaved population. Matthew Gilmore points to Brazil as "larger, more pervasive, and longer in duration," and he is unquestionably right on the second two points; and the number of Africans brought to Brazil is at least 8 times greater than to the British North American colonies/United States. But because of demographic increase I'm not sure the raw numbers are larger in Brazil. Does the basis for a reliable estimate exist for Brazil, or for the Sokoto Caliphate?

Robert,
you can find reliable figures on Brazil's enslaved population in "The Comparative Histories of Slavery in Brazil, Cuba, and the United States" by Laird Bergad. The whole book may be interesting to answer your question.

If Rob is asking about "largest in history," on the first page of Without Consent or Contract, Bob Fogel points out that "one high-water mark was reached during the first two centuries of the Roman Empire when, according to some estimates, three out of every four residents of the Italian peninsula -- 21 million people -- lived in bondage."  His notes point to variations in such estimates given the poor condition of the documentary record.  I don't have the book in front of me, but perhaps Orlando Patterson's Slavery and Social Death offers some guidance -- he examined about 80 slave societies for that work.

Peter Knupfer, Michigan State University

Dear Robert, the US-american slavery was, by 1860, the quantitatively largest slavery (4 million vs. 2.5-3 million in Brazil) in the Americas, but Brazil had until 1850 the largest Atlantic slave trade (around 2-2.5 Mio 1800-1851) and in Brazil (tropical) far more people died than in the USA (see: Tadman, Michael, “The Demographic Coast of Sugar: Debates on Slave Societies and Natural Increase in the Americas“, in: American Historical Review Vol. 105, Number 5 (Dec. 2000), S. 1534-1575). The most important (“modern” - not largest) slavery in the Americas (technical, industrial, technological, output, size of the plantations, richness of the owners, doctors, and different slave cultures) was, without doubt, Cuba (by 1841 more or less 450000 slaves). You may have a look at Dale Tomich’s works or at:
Review: A Journal of the Fernand Braudel Center, Binghamton University XXXI, no. 3 (2008) (=special issue edited by Dale Tomich & Michael Zeuske, eds., The Second Slavery: Mass Slavery, World-Economy, and Comparative Microhistories, Part II),
Zeuske, “The Second Slavery: Modernity, mobility, and identity of captives in Nineteenth-Century Cuba and the Atlantic World”, in: Laviña, Javier; Zeuske (eds.), The Second Slavery. Mass Slaveries and Modernity in the Americas and in the Atlantic Basin, Berlin; Muenster; New York: LIT Verlag, 2014 (Sklaverei und Postemanzipation/ Slavery and Postemancipation/ Esclavitud y postemancipación; Vol. 6), pp. 113-142.
The numbers of other slaveries (out of the Americas) you can find in German in: Zeuske, “Zahlen und Menschen: “mumbers games”?, in: Zeuske, Handbuch der Geschichte der Sklaverei. Eine Globalgeschichte von den Anfängen bis heute, Berlin/ Boston: De Gruyter, 2013, pp. 451-478.
Saludos, best, Michael

Robert Forbes mentioned Brazil, and the Sokoto Caliphate, and Peter Knupfer mentioned Rome. I don't know the numbers for those, but (apart from Britain, France and other slave-holding colonial-imperial states with huge numbers of slaves in their colonial territories -- which I also don't know the numbers for), also note that imperial China had enormous numbers of slaves.
In the words of James Watson, it “had one of the largest and most comprehensive markets for the exchange of people in the world” (p. 223 in James L. Watson, "Transactions in people: the Chinese market in slaves, servants, and heirs," in _Asian and African systems of slavery_, ed. James L. Watson, Berkeley: University of California Press, 1980, pp. 223–50 ).
He was writing mainly of late-imperial southern China. Basically, there was a range of different kinds of people bought and sold: Some of these would be instantly recognizable to us as slaves (in chains and all that), and some would be more in a grey area, held under the pretenses of patriarchal kinship, but also very much slave-like in relation to their masters. Estimates of actual numbers are difficult.
I included a discussion of these issues in my own article, "Slavery as the commodification of people: Wa 'slaves' and their Chinese 'sisters,'" Focaal: Journal of Global and Historical Anthropology 59 (2011), pp. 3-18.
I don't think that the global issues of comparability of historical numbers from overlapping categories have been adequately figured out yet, but slavery in the Chinese empire is underestimated in much of the literature, for sure.
Then there is India, Malaysia, etc.
yrs, Magnus Fiskesjö

In the 19th century, China may have been the largest. A small percentage of the total population, but with what was by far the world's largest population, there were a lot of slaves. Of course, there is the question of what is a system. Was Southeast Asia one system or many??

Good point.
The Chinese system extended far into Southeast Asia, as vividly demonstrated in Janet Lim's autobiography, Sold for Silver, and did not have clear borders as a system. As for Southeast Asia itself, I wrote "Malaysia" but slaving in that country (see f ex Dentan's marvelous book Overwhelming Terror) was of course not limited to the modern configuration of Malaysia but among Malay slaveholder across the archipelagos of Island Southeast Asia.
This makes delineating the units of comparison for answering the question of whether the US was "the largest slave system" so much more difficult. One could try to define slave systems as those governed & claimed as property by some sovereign power, as opposed to another, such as, with the slaves of the Thai realm or of the Burmese realm (though like in China, bondage comes in a range of various statuses that do not map easily onto US slavery).
In this sense, as I tried to point out, England and France were nodes in slave systems that extended across the Atlantic. How should they be delineated and the slaves counted? What's the percentage of slaves vs free in a "France-Haiti"?
The systems of the Indian Ocean or Southeast Asia or China are even more lacking in distinct borders, of the slave systems of each. A comparison might have to take all that into account. One might have to admit, there is not exact count. I haven't checked if any of the world slavery encyclopedias have tried to present such a measured comparison for global slavery.