From The Comanche Empire, by Pekka Hämäläinen (Yale U. Press, 2008), pp. 318-319:
It was an open secret that the livestock and laborers that fueled New Mexico’s economic growth during and after the Civil War years were looted from Texas and northern Mexico.
The contraband cattle and captive trade and the violence it fueled in Texas were a stinging embarrassment for the federal agents in New Mexico, Kansas, and Indian Territory. They had failed to restrain the Comanches, who ignored the reservation boundaries as defined in the Treaty of Little Arkansas, refused to relinquish slave traffic, and yet frequented Fort Larned, their assigned agency near the Big Bend of the Arkansas, to collect government supplies. Shameful reports of “lives taken and property stolen by Indians … fed and clothed and armed by the representatives of the U.S. Gov” poured out of Texas, putting enormous pressure on the Indian Office and its agents. Determined to extend emancipation from the South to the Southwest, federal agents repeatedly demanded that the Comanches and Kiowas relinquish their captives. But instead of eradicating slavery and captive trade, such interventions ended up supporting them. Comanches and Kiowas did turn numerous captives over to U.S. agents, but only if they received handsome ransoms in cash or goods. As one federal agent despaired: “every prisoner purchased from the Indians amounts to giving them a license to go and commit the same overt act. They boastfully say that stealing white women is more of a lucrative business than stealing horses.” The United States’ emancipation efforts had created a new outlet for slave trafficking for Comanches, and its punitive reconstruction policies in Texas opened a deep supply base: the demilitarized western part of the state lay wide open for Comanche slaving parties.
The struggle over the captives epitomized the collision between the Comanches and the United States and precipitated its progression to open war. The persistence of slavery and captive traffic convinced U.S. policymakers that the Southwest was not big enough for both traditional borderland cultural economics and the new American system of state-sponsored, free-labor capitalism. Perplexed and put off by their own involvement in the captive business, U.S. authorities, most of them Civil War veterans, started to call for tougher policies and, if necessary, the extermination of the slave-trafficking Indians. In 1867, when presented with the case of a thirteen-year-old Texas boy for whom Comanches demanded “remuneration,” General William Tecumseh Sherman, the commander of the U.S. Army, responded that the officials should no longer “Submit to this practice of paying for Stolen children. It is better the Indian race be obliterated.”