Final answer:
James K. Polk was the president responsible for expanding the US borders to incorporate the Southwestern region through the Mexican-American War and the Treaty of Guadalupe Hidalgo.
Step-by-step explanation:
The president who expanded the borders to include what is the Southwestern US today was James K. Polk. During his presidency, Polk was committed to territorial expansion under the banner of Manifest Destiny. He provoked a war with Mexico that led to the acquisition of vast territories through the Treaty of Guadalupe Hidalgo in 1848. This added crucial regions to the United States, including present-day California, New Mexico, Arizona, Nevada, Utah, and parts of Colorado and Wyoming. Polk's administration also saw a peaceful settlement with Great Britain regarding the ownership of the Oregon Country.