When we think of US slavery, we think of Africans being forced to work in the fields and the master's house. Some mainstream sources will give a nod to enslaved white people, although they're almost always labeled with the euphemism “indentured servants.” But you'll never hear about Native American slaves. Not because they didn't exist, but because they've been thrown into history's dustbin.
Columbus wasted little time before enslaving the native people on Hispaniola. The Spanish proceeded to make slaves out of the indigenous people all over the Caribbean, Latin America, Florida, and what is now the Southwestern US. The French did the same thing in Canada and Louisiana, as did the Portuguese in Brazil. From the early 1600s, the British were the primary enslavers of Indians in what we now call the Eastern US.
Native Americans were put to the lash in most colonies and territories east of the Mississippi River — including New York, Massachusetts, Rhode Island, Pennsylvania, Virginia, the Carolinas, Georgia — as well as the areas that were or became Arkansas, Kansas, Nebraska, Utah, New Mexico, and Arizona. A small number of Native Americans were taken from the West Indies to the American colonies, but by far most of the traffic was the reverse, with the mainland supplying the islands with Indian slaves to handle crops such as sugar.
How did the European colonists get their hands on Indian slaves? Many were captured directly during wars between Europeans and Native Americans. In some cases, Indians sold their children into slavery. In a third route, perhaps accounting for most slaves, Indian tribes sold their captives to the palefaces. As in Africa, slaves largely were not captured by Europeans but were snared by neighboring tribes, who then sold them to the white men. At first, Native Americans simply sold their prisoners of war, but tribes quickly started raiding other tribes for the sole purpose of snatching human chattel for the settlers.
The various European groups started using Indian slaves almost as soon as they set up settlements. The Yamasee war almost, but not quite, eliminated the practice in 1715 in the Southeast. The Spanish made slaves out of Indians in southern Mexico, continuing the practice as they pushed north of the Rio Grande, forcing their captives to work primarily in mines and as household servants. The practice slowed down a few years after the US wrested the Southwest from Mexico in 1848, but it definitely didn't stop. Even the Civil War didn't halt the practice. Immediately after the War Between the States, though, Congress took up the issue of Indian slavery in the Southwest, eventually directing William T. Sherman to liberate the captive Native Americans. (Although this effectively ended the slave trade, the practice of slaveholding didn't completely stop; a rancher in Arizona is known to have kept a female Apache, captured when she was fourteen, into the 1930s.)
In his scholarly book The Indian Slave Trade, history professor Alan Gallay notes: “Most Indian slaves were women and children, whereas the majority of African slaves were adult males.” Native Americans generally didn't make ideal slaves for various reasons. One of the main “problems” was that they could more easily fly the coop and rejoin their tribe or at least a friendly tribe. They also had a hard time resisting all those funky European diseases. Because of this, their numbers never came close to those of African slaves.
How many Native Americans were enslaved will never be known, so we have only very rough estimates. This trade wasn't documented nearly to the extent of its African counterpart, and most of the records that were created haven't survived. Professor Gallay has assembled what pieces remain to come up with a probable range of Indians who were enslaved by the British in the Southeast up to 1715: no less than 25,000 to 32,200, and no more than 51,000. In the seriously obscure book Indian Slave Trade in the Southwest, L.R. Bailey cites 6,000 slaves in the territory of New Mexico when the Civil War broke out.
So we're up to 31,000 to 57,000 without including any of New England, New York, the Southeast after 1715, antebellum New Mexico, the rest of the Southwest, the Plains states, or the Indians enslaved by Spaniards in Florida and the French in Louisiana. And that's only part of the hemispheric picture; native peoples enslaved in Canada, Central and South America, and the Caribbean islands obviously would add lots of misery to the total.