Final answer:
The U.S.-Mexican War was caused by the annexation of Texas and the territorial ambitions embodied in the concept of Manifest Destiny, both of which led to conflicts with Mexico.
Step-by-step explanation:
The causes of the U.S.-Mexican War during the era of Westward Expansion are deeply rooted in the concept of Manifest Destiny and the desire of the United States to acquire more land for expansion and influence. Two main events that led to the U.S.-Mexican War include the annexation of Texas and the subsequent dispute over its southern border, and the Manifest Destiny-driven ambition of the United States to acquire territories such as California and New Mexico. The annexation of Texas, which had been an independent republic where slavery was legal, brought the U.S. into direct conflict with Mexico.
Moreover, President James K. Polk's expansionist policies aimed to acquire further territories, reflecting a perception of Mexico as a weaker nation. These tensions ultimately led to the outbreak of the Mexican-American War.