It was actually the Triple Alliance versus the Texcalteca, the Mixteca, and the Huexotin:

"There were three lesser peoples to the southeast: the Texcalteca, of whom I have already spoken, with their capital at Texcala; the Huexotin with their capital at Huexotzinco; and the once mighty Tya Nuü—or Mixteca, as we called them—whose domain had shrunken until it consisted of little more than their capital city of Chololan. The first were our enemies, as I have said; the latter two had long ago been made our tribute payers and, like it or not, our occasional allies.

All three of those nations, however, like all three of ours in the Alliance, were being devastated by the Hard Times. After Motecuzóma's conference with his Speaking Council, he conferred also with the rulers of Texcóco and Tlácopan. Those three together drafted and sent a proposal to the three rulers in the cities of Texcala, Chololan, and Huexotzinco. In essence it said something like this:

"Let us all make war that we may all survive. We are diverse peoples, but we suffer the same Hard Times. The wise men say that we have only one hope of enduring: to sate and placate the gods with blood sacrifices. Therefore, we propose that the armies of our three nations meet in combat with the armies of your three nations, on the neutral plain of Acatzinco, safely far to the southeast of all our lands. The fighting will not be for territory, nor for rule, nor for slaughter, nor for plunder, but simply for the taking of prisoners to be granted the Flowery Death. When all participating forces have captured a sufficiency of prisoners for sacrifice to their several gods, this will be mutually made known amongst the commanders and the battle will end forthwith."

Gary Jennings, Aztec