Well, there were around 3000 Spanish conquistadors. Could they really conquer the whole Aztec empire (5 mln people) without alliances with local tribes?
Without local tribes? Probably not, but that’s almost always how conquering actually happens (by exploiting existing fault lines). The situation with Alexander the Great is kind of representative. Alexander the Great had an army of about 30,000 people and conquered the Persian Empire which had a population of about 50 million. The Conquistadors had 3000 and conquered the Aztec Empire which had a population of 5 million, although the Conquistadors also had the benefit of disease traveling before them and not just better tactics but also far superior metallurgy. It doesn’t necessarily take an enormous advantage to conquer large territories, and the Conquistadors had numerous advantages.
The biggest advantage the Conquistadors has was everyone else in the area fucking hated the Aztec. They were horrible to have as neighbors and when any chance to fuck them over, the Spanish, came everyone jumped on board.
That was my point above that everyone seems to have missed. Even if diseases wiped out 90+% of the local population, they would still greatly outnumber the conquistadors. So it wasn't purely a balance of manpower. Sure, the diseases weakened the resistance, but it wasn't the deciding factor. Diamond says as much in his book.
Disease was still very much the deciding factor. The europeans had an advantage in technology, but not a staggering one. Cortez's forces were successfully defeated by the Aztecs on more than one occasion, and it was only after disease wiped out a large portion of Tenochtitlan's population and a protracted siege that Cortez and his allies could wear down their defenses enough to seize the city. When Pizarro entered Peru, the area had already been severely depopulated by disease and civil war, the civil war itself being kicked off by the death of both the Emperor and his heir dying of small pox. He convinced the winner of that civil war to visit him unarmed and then captured him and masscred his retinue, and used his hostage to get the Inca generals to stand down. When the Incas eventually rebelled, they too were successful in the field against the spanish - Manco Inca managed to wipe out 4 relief columns sent to break his Siege of Cuzco. Guerilla tactics were common but Manco also defeated the Spanish in open battles, such as the Battle of Ollantaytambo. While Manco was ultimately unsuccessful in seizing control of Cuzco, the Spanish likewise were unable to defeat him, and his Neo-Inca state survived for decades.
