What was the incas major weaknesses?

Incas were isolated in the "new world" and didn't have contact with the other cultures around the world like Asians, Europeans and Africans. For example, the Chinese are the ones who invented gun powder and the idea spread throughout the rest of the world and was eventually used to propel ammunition (guns). 
The isolation also made Incas susceptible to European diseases which they had never been exposed to. A large percentage of those populations died and were weakened before the fighting even began. They also didn’t have horses or most common beasts of burden because they simply didn’t exist in South America. The Incas were one of the most powerful empires in their respective regions and like most empires, they had their share of enemies. Hernán Cortés formed allies with neighboring tribes which played a huge role in defeating the Aztecs, Francisco Pizarro did the same in South America.

the united states began searching for a way to reduce oil

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