Final answer:
In the 1840s, the U.S. pushed into the Oregon Territory, indicating to Mexico its intent to expand territorially. These actions contributed to the occupation of Mexico City and the signing of the Treaty of Guadalupe Hidalgo, which led to the U.S. acquiring much of Mexico's northern lands. The territorial expansion further intensified the national debate on slavery.
Step-by-step explanation:
In the 1840s, the United States made significant movements into the Oregon Territory. Thousands of American settlers streamed into the region via the Oregon Trail. This settlement helped strengthen U.S. claims against British interests in the area. Under President James K. Polk, known for his expansionist views, this signaling to the Mexican populace was clear: the U.S. had ambitions to expand its borders across the continent, embracing the idea of Manifest Destiny.
Not content with just Oregon, Polk had eyes on the Mexican provinces of New Mexico and California. While Polk was prepared to use force, he initially sought to acquire these territories peacefully. The negotiations with Great Britain that established the 49th parallel as the northern U.S. border preemptively secured U.S. interests in the northwest, allowing Polk to focus on Mexico. This action signaled to Mexico that the U.S. was determined to expand and could potentially use military force to do so.
The culmination of these expansionist actions was the U.S. Army's occupation of Mexico City in 1848 and the subsequent Mexican cession of territories through the Treaty of Guadalupe Hidalgo. As a result, Mexico ceded to the U.S. a vast amount of territory, including what would become California, Arizona, New Mexico, Nevada, Utah, and parts of Colorado and Wyoming. This shift of North American power dynamics was part of a larger strategy by the U.S. to advance its territory from coast to coast.
The immediate effect of the U.S. actions in the Oregon Territory and the acquisition of Mexican land sparked a series of contentious debates and conflicts within the United States, particularly over the issue of slavery. The Compromise of 1850 and the reactions to the Fugitive Slave Law highlighted the deep schisms within the U.S. that would ultimately lead to the Civil War.