From The Comanche Empire, by Pekka Hämäläinen (Yale U. Press, 2008), pp. 144-145:
A more subtle but ultimately more serious challenge to the Comanche-Spanish emerged in the late 1790s, when American merchants and agents operating out of Spanish Louisiana began to push into the southern plains. Evading Louisiana’s Spanish officials—and sometimes cooperating with them—itinerant American traders infiltrated the contested borderland space between Spanish Texas and the United States and then proceeded toward eastern Comanchería. Americans’ arrival constituted a litmus test for the pact between eastern Comanches and Texas, for the treaty of 1785 had anticipated the United States’ westward thrust and explicitly prohibited Comanches from dealing with American agents. Spanish officials expected eastern Comanches to honor the treaty, remain loyal to Texas, and banish the intruders. They expected that not only because Comanches had signed a political contract but also because Spanish gifts and generosity obliged them to do so.
The Americans, however, did not come as conquerors carrying guns and banners but as merchants carrying goods and gifts, and eastern Comanches eagerly embraced them as potential trading partners. Comanches simply viewed the linkage between presents and politics differently from Spaniards. Gifts, Bourbon administrators insisted, were contractual objects that created a political bond, an exclusive bilateral union, whereas for Comanches the meaning of gifts was primarily of a social nature. Bourbon officials insisted that Spanish gifts should forbid Comanches from trading with foreign nations, but this was a narrow interpretation of loyalty and friendship that did not easily translate into the Comanche worldview. If foreigners—American, French, or any other kind—who entered Comanchería were willing to adhere to Comanche customs and expectations, Comanches had no reason to reject them. Indeed, as the pages that follow will show, by demanding eastern Comanches to choose between devotion to Spain and hospitality to Americans, Texas officials eventually wrecked their alliance with the Comanche nation.
And so, by simply letting American newcomers in, eastern Comanches began to turn away from their fledgling, uneasy alliance with Spain, and toward American markets and wealth. It was a momentous shift that changed the history of the Southwest. By establishing exchange ties with Americans, and by linking their pastoral horse-bison economy to the emerging capitalist economy of the United States, eastern Comanches set off a sustained commercial expansion that eventually swept across Comanchería. Spanish officials were slow to recognize this change and even slower to react to it. When José Cortés applauded Comanches’ loyalty to Spain in 1799, eastern Comanches were already engaged in an active trade with the westering Americans, and when Pino echoed Cortés’ praise thirteen years later, eastern Comanches had already turned their rancherías into a thriving gateway between the Southwest and the U.S. markets. By the time the Spanish colonial era came to an end in 1821, the entire Comanche nation had moved out of the Spanish orbit. They commanded a vast commercial empire that encompassed the Great Plains from the Río Grande valley to the Mississippi and Missouri river valleys, and they looked to the north and east for markets, wealth, allies, and power.