What ideology justified the territorial expansion of the United States in the 19th century?

The ideology that justified the territorial expansion of the United States in the 19th century is known as Manifest Destiny. This concept emerged in the 1840s and was rooted in the belief that the expansion of the United States across the North American continent was both justified and inevitable. Advocates of Manifest Destiny believed that Americans were destined by God to spread democracy and capitalism across the continent, which they viewed as a moral obligation.

Manifest Destiny played a significant role in motivating various territorial acquisitions during this period, including the annexation of Texas, the Oregon Trail migrations, and the acquisition of California following the Mexican-American War. The belief also encompassed broader cultural notions of American exceptionalism, positioning the United States as a beacon of freedom and progress.

In contrast, isolationism prioritizes keeping a nation out of foreign affairs, colonialism involves the establishment of control over foreign territories, and industrialization pertains to the economic development of industries and technologies—not directly to the justification for territorial expansion. Therefore, the correct answer, Manifest Destiny, encapsulates the specific ideology that fueled the expansionist policies of the United States during the 19th century.