What did "Manifest Destiny" signify for the United States in the mid-1800s?

"Manifest Destiny" in the mid-1800s was primarily about the belief that the United States was destined to expand its territory across the entire North American continent, from the Atlantic Ocean to the Pacific Ocean. This ideology was fueled by a sense of cultural and racial superiority and the notion that expansion was both justified and inevitable. The desire for continental expansion from East to West was a driving idea of the time, leading to significant events like the Louisiana Purchase, the annexation of Texas, and the Oregon Trail migration.

The concept encapsulated the American ethos of pioneering and was tied to economic motives, as access to new lands was seen as a pathway to prosperity. While the other choices contain elements related to U.S. history during that period, they do not capture the essence of "Manifest Destiny" as a narrative of continental ambition and the societal belief in expansionist destiny. This is why the correct choice revolves around the fundamental goal of extending the nation's borders across the continent.