Ancient history

One reason the Spanish conquistadors were able to conquer Aztec and Inca Empires rapidly is that?

The spread of diseases such as smallpox.

The Aztecs and Incas had never come into contact with diseases like smallpox or measles before, and they had no natural immunity. These diseases spread quickly and often left entire populations decimated, making it easier for the Spanish to defeat these once-formidable armies.