If you want the Incas to survive, find a way of avoiding the destructive civil war that was raging when the Spanish first arrived. That way, the Spaniards who come to Peru encounter a strong, united country under one ruler. This makes it more likely that they will be beaten back.
Secondly, there's no way to avoid the epidemics which decimated the Inca in real life. Even if we remove the most common varient of smallpox and replace it with something less serious, there are still a wide variety of souped-up Eruropean diseases just waiting to be unleashed on the natives of the Americas. If smallpox doesn't kill Huayna Capac, measles will.
What you really need is for this pestilence to hit the Incas quietly, at a time when there are no Europeans around to take advantage. Whatever happens, the empire is going to be demographically crippled for decades. The best that can be hoped for is that the Spanish don't use it to their advantage.
Suppose that things pan out as in OTL, but Pizzaro never catches that gold-laden trade barge. He hacks his way down to the north of Ecuador, not knowing what exactly it is he is looking for, but never makes it: some horrible fate befalls the Spanish at the hands of the natives before they can penetrate too deeply into the empire. Meanwhile, smallpox and a range of other European disease push down the Andean coast, killing hundreds of thousands of Incas. The royal family is decimated, leaving Huascar sole, undisputed ruler of the Empire (Atahualpa dies of flu).
In the face of the mass-die off of his family, Huascar hears reports about an aggressive invasion of viracochas from the north, which was cut to pieces by the garrison in Quito as it attempted to cross the Empire's boundaries. Not, however, before inflicting massive casualties on the soldiers who opposed them.
The Emperor makes the leap of logic which allows him to conclude that the Spaniards and the diseases are linked, and that the Spanish are, therefore, a very bad thing. As a result, the Empire goes into a type of lockdown, with European encroachment met by massive Incan armies who know, to some extent, what to expect from these mysterious harbringers of doom. There's no misplaced trust in these scenarios: the Spanish are killed on sight.
If Huascar and his descendents can keep this up for a hundred years, they get the one thing that they didn't have in real life, which was time. I see no reason why they wouldn't adopt the use of firearms and horses if they find a way to access them. Eventually, it may well be that another European power, attempting to reach the Inca by more peaceful means, could give them assistance in modernising their armies in return for trade concessions. (God knows the Spanish had enough enemies). England would be a good candidate.
In such a scenario, the Empire continues, weakened demographically but revitalised technologically. With access to firearms and steel weapons, the Incas can fight the Spanish on a more equal basis. Even allowing for the inevitable demographic colapse, an Inca Empire with the knowledge of what it is fighting against is going to be a formidable foe, particularly for the periodical 100-200 man expeditions the Spanish will be throwing at them. Eventually, it may well come to a point where the Spanish simply throw in the towel, and content themselves with expansion elsewhere.
The Inca Empire of this timeline, then, remains backward, and is periodically ravaged by epidemics. It is, however, acquiring knowledge about the rest of the world rapidly, so there is no longer the same 'element of surprise' which Pizzaro deployed so effectively in OTL. The fact that the empire doesn't fall has massive, massive butterflies - no Inca gold flows into Spain, just for starters.
All this is very hypothetical, of course and, more to the point, very unlikely. Sad to say, the Spanish were almost inevitably going to ahve the upper hand in this particular fight.
Secondly, there's no way to avoid the epidemics which decimated the Inca in real life. Even if we remove the most common varient of smallpox and replace it with something less serious, there are still a wide variety of souped-up Eruropean diseases just waiting to be unleashed on the natives of the Americas. If smallpox doesn't kill Huayna Capac, measles will.
What you really need is for this pestilence to hit the Incas quietly, at a time when there are no Europeans around to take advantage. Whatever happens, the empire is going to be demographically crippled for decades. The best that can be hoped for is that the Spanish don't use it to their advantage.
Suppose that things pan out as in OTL, but Pizzaro never catches that gold-laden trade barge. He hacks his way down to the north of Ecuador, not knowing what exactly it is he is looking for, but never makes it: some horrible fate befalls the Spanish at the hands of the natives before they can penetrate too deeply into the empire. Meanwhile, smallpox and a range of other European disease push down the Andean coast, killing hundreds of thousands of Incas. The royal family is decimated, leaving Huascar sole, undisputed ruler of the Empire (Atahualpa dies of flu).
In the face of the mass-die off of his family, Huascar hears reports about an aggressive invasion of viracochas from the north, which was cut to pieces by the garrison in Quito as it attempted to cross the Empire's boundaries. Not, however, before inflicting massive casualties on the soldiers who opposed them.
The Emperor makes the leap of logic which allows him to conclude that the Spaniards and the diseases are linked, and that the Spanish are, therefore, a very bad thing. As a result, the Empire goes into a type of lockdown, with European encroachment met by massive Incan armies who know, to some extent, what to expect from these mysterious harbringers of doom. There's no misplaced trust in these scenarios: the Spanish are killed on sight.
If Huascar and his descendents can keep this up for a hundred years, they get the one thing that they didn't have in real life, which was time. I see no reason why they wouldn't adopt the use of firearms and horses if they find a way to access them. Eventually, it may well be that another European power, attempting to reach the Inca by more peaceful means, could give them assistance in modernising their armies in return for trade concessions. (God knows the Spanish had enough enemies). England would be a good candidate.
In such a scenario, the Empire continues, weakened demographically but revitalised technologically. With access to firearms and steel weapons, the Incas can fight the Spanish on a more equal basis. Even allowing for the inevitable demographic colapse, an Inca Empire with the knowledge of what it is fighting against is going to be a formidable foe, particularly for the periodical 100-200 man expeditions the Spanish will be throwing at them. Eventually, it may well come to a point where the Spanish simply throw in the towel, and content themselves with expansion elsewhere.
The Inca Empire of this timeline, then, remains backward, and is periodically ravaged by epidemics. It is, however, acquiring knowledge about the rest of the world rapidly, so there is no longer the same 'element of surprise' which Pizzaro deployed so effectively in OTL. The fact that the empire doesn't fall has massive, massive butterflies - no Inca gold flows into Spain, just for starters.
All this is very hypothetical, of course and, more to the point, very unlikely. Sad to say, the Spanish were almost inevitably going to ahve the upper hand in this particular fight.
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