What was the cortez's unknown secret weapon and why didn't he know about it? (site 2)

His unknown secret weapon was disease.
When Cortez and his army invaded aztecs, a couple of his men were infected with smallpox.
Since the aztecs did not possess any immunity for it, the majority of their people were weakened by the disease, which make it easier for cortez army to conquer their region.

The holy roman empire.