What was the primary idea behind Manifest Destiny in the 1840s and 1850s?

The primary idea behind Manifest Destiny in the 1840s and 1850s was that the expansion of the United States across the North American continent was a divine right and duty. Adherents of Manifest Destiny believed that it was preordained by God for the U.S. to expand its territory, thereby spreading democracy and civilization. This belief was deeply intertwined with a sense of American exceptionalism, suggesting that the nation had a unique mission to lift and enlighten other nations and cultures.

The other options do not capture the essence of Manifest Destiny as accurately. While military conquest did occur during this period, it was more often justified under the belief that expansion was divinely sanctioned rather than being the sole motivation. The notion that expansion was solely for trade fails to account for the ideological and spiritual dimensions of the belief. Lastly, the idea of establishing international alliances does not align with the primary motivations of Manifest Destiny, which emphasized a unilateral expansionist policy rather than cooperative international relationships. Thus, the reasoning behind Manifest Destiny was rooted in the belief of divine providence guiding American expansion.