Which war resulted in the United States gaining California and Arizona?

The Mexican-American War, which took place from 1846 to 1848, resulted in the United States acquiring significant territories, including California and Arizona. This conflict stemmed from various factors, such as the U.S. annexation of Texas and disputes over its southern border. As a result of the war, the Treaty of Guadalupe Hidalgo was signed in 1848, which formally ended the conflict and led to the U.S. purchasing territories that now encompass not only California and Arizona but also parts of New Mexico, Nevada, Utah, and Colorado.

The other wars listed did not contribute to the territorial expansion that included California and Arizona. The War of 1812 primarily involved issues of maritime rights and territorial disputes in Canada. The Civil War was primarily a conflict over slavery and states' rights, without any direct implications for territorial gains. The Spanish-American War, occurring in 1898, involved a different set of territorial acquisitions, mainly in the Caribbean and the Pacific, without directly resulting in the U.S. gaining California and Arizona. Thus, the Mexican-American War is recognized as the key event leading to the U.S. obtaining these western territories.