The era of expansionism in the United States gained great momentum in the 1840s and saw the finalization of the boundaries of the continental United States. Manifest Destiny justified expansion with a sense of mission and purpose. It portrayed American expansion as inevitable, just, and divinely foreordained. Expansion added Texas and Oregon to the Union and was an underlying cause of war with Mexico, which resulted in the acquisition of vast territories in the Southwest, including California. Although it was a popular movement, it further antagonized the divisions between free and
slaveholding states.

As the country grew and incorporated more and more territory, the delicate balance established by the Missouri Compromise became increasingly tenuous. Finally, the era of Manifest Destiny profoundly influenced foreign relations, as some of the great European powers such as Great Britain reevaluated their opinion of U.S. military strength, and Mexico and much of Latin America came to regard the United States with increasing suspicion. When California applied to the Union as a free state, both sides felt compelled to press their interests at the national level. The Compromise of 1850 resolved the question of California’s status, though it hardly lessened the tensions that existed between those who favored and those who opposed slavery.