Spanish authorities feared African-Indian alliances would prove impossible to subdue

Evidence

[I]t appears, Our Majesty, that the time is coming when these people will have become masters of the Indians, inasmuch as they were born among them and their maidens, and are men who dare to die as well as any Spaniard in the world. But if the Indians become corrupt and join with them, I do not know who will be in the position to resist them. It is evident that this mischief will take place in several years. … By the 1560s fugitive slaves from the mines of the north were terrorizing the regions from Guadalajara to Zacatecas, allying with the Indians and raiding ranches. In one case maroons from the mines of Guanajuato joined with unpacified Chichimec Indians in a brutal war with the settlers. The viceroy was informed that they were attacking travelers, burning ranches, and committing similar “misdeeds.” To the east, slaves from the Pachuca mines took refuge in an inaccessible cave from which they sallied forth periodically to harass the countryside. Negroes from the Atotonilco and Tonavista mines joined them with arms, and created an impregnable palenque. — Cedric J. Robinson, Black Marxism_ The Making of the Black…, 648