From Seven Myths of the Spanish Conquest, by Matthew Restall (Oxford, 2004), Kindle Loc. 3181-3207:
Guns, too, were of limited use. Cannons were few in number in the Americas, and without roads or navigable rivers, their transportation was a major challenge. Much of the Americas where Spaniards fought was tropical or subtropical, and in the humidity the powder became too wet to fire. Firearms, in the form of harquebuses, whose unwieldy barrels required the support of tripods, were likewise not plentiful and required dry powder. Vargas Machuca advocated Spaniards using harquebuses in the Americas, but his detailed exposition on how to avoid damaging the gun, getting it wet, or discharging it prematurely or by accident would surely have caused any conquistador to think twice about carrying such a weapon. The more reliable and faster-loading musket was not invented until decades after Cortés and Pizarro invaded the American mainland. Nor had Europeans yet developed volley-fire techniques, in which soldiers formed banks of rows in order to provide continuous fire, although there were seldom enough firearms in a Conquest company to have made good use of such a technique. Those Spaniards who did have firearms were lucky to get a single shot off before reversing the weapon to use as a club or dropping it to concentrate on sword wielding.
The one weapon, then, whose efficacy is indubitable was the steel sword. It alone was worth more than a horse, a gun, and a mastiff put together. Because a steel sword was longer and less brittle than the obsidian weapons of Mesoamerican warriors, and longer and sharper than Andean clubbing weapons or copper-tipped axes, a Spaniard could fight for hours and receive light flesh wounds and bruises while killing many natives. Spanish swords were just the right length for reaching an enemy who lacked a similar weapon. Pizarro preferred to fight on foot so he could better manipulate his sword. Descriptions of battles in which Spanish swordplay caused terrible slaughter among native forces pepper the Conquest accounts of Cieza de León, Cortés, Díaz, Gómara, Jerez, Oviedo y Baños, Zárate, and others. Military historian John Guilmartin deftly summarizes the point: “While Spanish success in combat cannot be attributed to a single factor, it is clear that the other elements of Spanish superiority took effect within a tactical matrix established by the effectiveness of Spanish hand-held slashing and piercing weapons.”
This trilogy of factors—disease, native disunity, and Spanish steel—goes most of the way toward explaining the Conquest’s outcome. Remove just one and the likelihood of the failure of expeditions under Cortés, Pizarro, and others would have been very high. As Clendinnen has observed of the Spanish-Mexica war, both Spaniards and natives were aware that the Conquest was “a close-run thing,” a point that applies broadly across the Conquest. The failed expeditions outnumbered successful ones, and cautionary tales can be found by looking at the fate of Spanish expeditions such as Montejo’s early attempts to conquer Yucatan, the early campaigns into Oaxaca’s northern sierra, or the Pizarro-Orellana journey into Amazonia. Spaniards would have suffered steady mortality from fatal wounds, starvation, disease, and so on, with survivors limping back to Spain or to colonial enclaves scattered along the coasts and islands. Time and again, this outcome was averted because Spanish steel weapons permitted them to hold out long enough for native allies to save them, while the next wave of epidemic disease disrupted native defenses.