At that time , the massive casualties on the Native's happened because the Spanish army has a superior arsenals compared to the native American Tribes.
On top of that, the The spanish also became a carrier of various diseases (such as smallpox) and infected the native american population. Since their medical knowledge was very limited at that time, the disease manage to wiped out a lot of their population. Eventually, the battles made some tribes such as Calusa and Timucua became extinct.
