Final answer:
During the Mexican-American War, the United States, driven by expansionistic ideas of Manifest Destiny, intentionally entered into a conflict with Mexico which led to the U.S. acquisition of a large part of Mexico's territory.
Step-by-step explanation:
The statement that best describes the actions of the United States during the Mexican-American War is that the U.S., motivated by Manifest Destiny and expansionist desires, provoked a war with Mexico. The U.S. insisted that the Rio Grande was the border between Mexico and itself, despite the long-established boundary at the Nueces River. This lead to a tense confrontation in the disputed territory which resulted in a series of engagements that President Polk used to justify a call for war. Following the war, in which the U.S. military occupied Mexico City, Mexico was forced to sign the Treaty of Guadalupe Hidalgo, ceding a vast amount of territory to the United States, an area that now encompasses many southwestern states.
President Polk had a clear aim to extend U.S. territory and charged John Slidell with secretly attempting to acquire Mexican northwest lands. Resistance from Mexico led to Polk sending troops into the disputed area, leading to military skirmishes that were leveraged as a casus belli. The United States emerged victorious from the conflict, significantly expanding its territory and fulfilling its continental aspirations at Mexico's expense. However, this victory also fanned the flames of debate over slavery in the newly acquired territories and altered the U.S.'s relationship with Latin American countries.