Answer:
The country that had settlements established in the Southwest United States in the 1500s was Spain.
Step-by-step explanation:
The territory that currently forms part of the American Southwest (the states of Arizona, California, Colorado, Nevada, New Mexico, Utah, Oklahoma and Texas) was part of the colony of New Spain, which lasted from 1535 to 1821, when Mexico became independent. Later, these territories were acquired by the United States as a result of the Mexican-American War.