What historical conflict contributed significantly to the establishment of American border expansion policies?

The Mexican-American War was a pivotal conflict that played a crucial role in the establishment of American border expansion policies. Fought between 1846 and 1848, the war arose from disputes over territory and borders following the annexation of Texas. The conflict resulted in significant territorial gains for the United States, as the Treaty of Guadalupe Hidalgo ceded vast areas, including present-day California, Arizona, New Mexico, Nevada, Utah, and parts of Colorado and Wyoming, to the United States.

This expansion was not just about acquiring land; it embodied the concept of Manifest Destiny, the belief that it was the United States' destiny to expand across the North American continent. The war and the subsequent land acquisition encouraged settlers to move westward, facilitated the spread of American culture and institutions, and ultimately shaped future border policies. The successful outcome of the war reinforced the idea that military actions could achieve territorial and political goals, shaping the nation's approach to expansion for decades to come.

The other conflicts mentioned, while significant in their contexts, did not have the same direct impact on the policies governing territorial expansion. The Aroostook War dealt primarily with a border dispute between Maine and Canada without the same scale of territorial change. The Nebraska-Kansas Conflict was significant for