I think the answer that you're looking for is Manifest Destiny. If its not, than I am sorry! Lol!
Answer:
The correct answer is A. The introduction of the horses by the spanish.
Explanation:
The culture of the Plains Indians changed by the introduction of horses by the Spanish.
The Indians in the Americas did not have horses. These animals were brought by the Spanish conquerors. So when the Spanished conquered territories in the Americas, the use horses as they did in Spain. This really benefitted the Indians in many ways. The obvious one, transportation. Horse ride faster than any other known method by the Indians in the Plains. And they could carry more things that a human. Just let's remind that Aztec's servants used to run 500 kilometers from the city of Tenochtitlan(nowadays modern Mexico City) to the port of Veracruz, to bring Emperor Monctezuma fresh fish to eat.
The United States planned to defeat the Viet Cong using search-and-destroy missions. U.S. troops would leave base, engage the Viet Cong where they found them, and then return to base.
