Disease was the number one killer of native peoples. They had had little exposure to the diseases of Europe. This led to some villages being almost completely wiped out and some tribes destroyed.
Answer:
Because of his lenient behavior towards Native Americans, he strongly disliked them, and therefore it made him less popular, and a not very effective president.
It was "a. refrigerated rail cars" that had the greatest impact on cattle ranchers on the Great Plains, since this new technology allowed for beef to be sent to further distances than every before.