D. took control of the territory that now makes up the Southwest.
The Treaty of Guadalupe Hidalgo, signed in 1848, ended the Mexican-American War and resulted in significant territorial changes. As a result of the treaty, the United States acquired a vast amount of territory, including present-day California, Nevada, Utah, Arizona, New Mexico, Colorado, and parts of Wyoming, Kansas, and Oklahoma. This territory, known as the Mexican Cession, became part of the United States, and it is the Southwest region of the country today.