Final answer:
The actions of Hernando DeSoto and other conquistadors led to disease outbreaks and population decline among the Native Americans due to their lack of immunity to European diseases.
Step-by-step explanation:
The actions of Hernando DeSoto and other conquistadors towards the Native Americans led primarily to disease outbreaks and population decline. When the Spanish arrived in the Americas, they brought with them diseases to which Native Americans had no immunity, like smallpox.
Due to the lack of previous exposure and genetic immunity, these diseases spread rapidly among indigenous populations, leading to catastrophic effects. The European diseases were particularly devastating, with estimates suggesting that up to 90 percent of the Native American population perished as a result, which greatly eased the conquest for the European invaders.