Manifest Destiny was a widely believed ideology in the 19th century stating that it was the destiny of the United States to expand its territory from coast to coast. This idea of territorial expansion was deeply rooted in notions of American exceptionalism and the belief in the moral rightness of spreading American values and institutions. Manifest Destiny played a significant role in shaping the expansionist mindset during the era of Westward Expansion.

One notable proponent of Manifest Destiny was President James K. Polk, who sought to extend American control over the entire continent. Under his administration, the United States saw remarkable territorial gains through key events such as the Mexican-American War. The Treaty of Guadalupe Hidalgo in 1848, which ended the war, resulted in the acquisition of large portions of present-day states including California, Nevada, Utah, Arizona, and parts of Colorado, New Mexico, Wyoming, and Oklahoma.

Frederick Jackson Turner's Frontier Thesis further influenced the perception of the American West and the frontier. In his famous 1893 essay, Turner argued that the frontier was central to shaping the American identity, promoting democratic values, and fostering individualism. According to Turner, the closing of the frontier in the late 19th century marked a significant turning point in American history, leading to the need for new sources of growth and expansion.

While Manifest Destiny provided a moral and ideological justification for Westward Expansion, it also had its detractors. Critics questioned the notion of American exceptionalism and raised concerns about the moral implications of forcibly displacing and subjugating indigenous populations. Some argued that Manifest Destiny was used as a pretext for aggressive territorial expansion and the establishment of new slave states, heightening tensions between the North and the South.

Despite these debates and criticisms, Manifest Destiny and the Frontier Thesis remain crucial in understanding the motivations and mindset behind Westward Expansion during the 19th century.