The enemy of my enemy is my friend

From Carnage and Culture by Victor Davis Hanson. Page 211.

The distinguished Aztec scholar Ross Hassig has rightly pointed out that most narratives of the conquest underplay the Mesoamerican contribution to the Spanish victory. So let us be clear: Cortés could not have conquered Tenochtitlán within a mere two years without vast support of native allies (initially the Totonacs and later the Tlaxcalans); nor could the surrounding Indians, who had fought the Aztecs in vain for decades prior to the European arrival, have destroyed the Aztec capital without the support of Cortés. The answer in assessing the critical role of the native involvement is one of degree, and involves the question of time and cost.

The tens of thousands of Indians who, as warriors, porters, and construction workers, aided, fought alongside, and fed Cortés were indispensable to the Castilians’ effort. Without their assistance Cortés would have required thousands of Spanish reinforcements and lost hundreds more men in an effort that might have taken a decade or more. Nevertheless, he would have accomplished his conquest even had he battled a united Mexico without native assistance. The Spanish conquest of Mexico— against populations without horses, the wheel, steel or iron weapons, oceangoing ships, gunpowder weapons, and a long tradition of scientific siegecraft—is emblematic of a systematic pattern of brutal conquest of the New World that elsewhere did not necessarily demand native complicity.

The Mesoamericans fought the Aztecs not because they were enamored of the Spanish—indeed for much of 1519 and early 1520 they tried to exterminate Cortés—but because they met an unexpected and powerful enemy who could be unleashed on their even greater adversary, Tenochtitlán, which had systematically butchered their own women and children in a most gruesome and hideous fashion. The near constant wars of the past century with the Aztecs had left most Mesoamerican peoples between the interior and the coast—the Tlaxcalans especially—under either an oppressive subjugation that stripped their fields and often their population for material and human tribute, or a state of siege for as much as six months out of the year to ward off Aztec depredations.

The appearance of the Spanish convinced most of the subjects of the Aztec empire that here was a people whom they could not defeat, yet who could annihilate their archenemies, the Mexicas, and possessed such technological and material advantages—as the prescient Aztec defenders reminded the Tlaxcalans during the last bitter days of the siege—as to be able to establish a lasting hegemony over all the natives of Mexico. We should see the indigenous contribution as the fuel that fed the fire that consumed the Aztecs, but concede the spark and flame to be all Spanish. Without the Spaniard presence even the brave Tlaxcalans would not have freed—and heretofore had never freed—themselves from Aztec oppression. Given the Western ability to produce deadly weapons, its propensity to create cheap, plentiful goods, and its tradition of seeing war in pragmatic rather than ritual terms as a mechanism to advance political ends, it is no surprise that Mesoamericans, African tribes, and native North Americans all joined European forces to help kill off Aztecs, Zulus, and Lakotas.