The correct answer is - The Europeans brought diseases like smallpox that killed many of the native people and weakened the tribes.
Unlike the popular belief that most of the native population was killed in battles with the Europeans, the native populations active died out because of diseases, like the smallpox for example.
The Europeans brought many things with them, and one those things were the diseases. The native populations didn't had the immune system that can deal with those diseases and they were fatal towards them. It is estimated that around 95% of the native populations died because of diseases brought by the Europeans.