Which war is associated with the struggle for U.S. territorial expansion in the 1800s?

The Mexican-American War is a pivotal event in U.S. history specifically tied to the nation's quest for territorial expansion during the 1800s. This conflict, which occurred between 1846 and 1848, stemmed from U.S. annexation of Texas and disputes over its southern boundary. The war resulted in significant territorial gains for the United States, including parts of present-day California, Arizona, New Mexico, Colorado, Utah, and Nevada, further solidifying the concept of Manifest Destiny—a belief that the U.S. was destined to expand across the continent.

The other conflicts mentioned are not associated with this specific period of territorial expansion. World War I, occurring from 1914 to 1918, was mainly about global alliances and imperial conflicts rather than U.S. land acquisition. The Korean War took place in the early 1950s and was focused on the division of Korea during the Cold War, while the Spanish-American War occurred in 1898 and primarily involved the U.S. intervening in Cuba and acquiring territories like Puerto Rico and the Philippines rather than pivotal territorial expansions in the continental U.S. Both of these later wars do not align with the intense period of westward expansion observed in the 1800s.