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That would make it perhaps the biggest empire in all of the indigenous Americas before the Spanish arrival.
Recent archeological research suggests that the Incas actually had a fairly potent state-level society in the Cuzco area that took about 200 years to develop and that the empire as an expansionist polity probably started early in the 15th century, that is, sometime fairly soon after 1400.
The question is, how much of the organizational capacity for the expansion developed before Pachacuti came along?
So the local conflict made it difficult for societies of the highlands to ally with one another and resist the Inca advances.
Another thing that contributed to the Inca advancement was that conflict had long been honed as a requisite for leadership.
As a consequence of this, it was really a battle for the strongest, most able person to lead the society.
That's a very effective way to create leadership in a people who were trying to expand their power.
The Incas didn't really have major competitors comparable to their own power until they ran into the Chimíº on the north coast of Peru, who actually had an empire.