What event's result caused the United States to acquire California and New Mexico?

The acquisition of California and New Mexico by the United States was a direct result of the Mexican-American War, which occurred from 1846 to 1848. This conflict was rooted in the U.S. desire to expand its territory under the concept of Manifest Destiny, which held that the nation was destined to expand across the continent.

The war began after disputes over the annexation of Texas and the boundaries between Texas and Mexico. Following military confrontations and a series of victories by U.S. forces, the war concluded with the Treaty of Guadalupe Hidalgo in 1848. This treaty formally ended the war and mandated that Mexico cede a large portion of its northern territories to the United States, including what would become California and New Mexico, as well as parts of Arizona, Colorado, Nevada, and Utah.

This territorial expansion significantly shaped the future of the United States, contributing to its rise as a continental power and intensifying the debates over slavery in these new territories. It is this war that is specifically tied to the acquisition of the southwestern lands, making it the correct answer. Other events listed, such as the American Revolution, Civil War, and Spanish-American War, do not directly relate to the U.S. acquiring California and New Mexico.