The war with Mexico and the annexations of Mexican territory that followed were believed by most Americans to be the result of their "manifest destiny." The term is generally attributed to John L. O'Sullivan, a Democratic newspaper editor from New York. O'Sullivan's conception of manifest destiny came to national attention through an editorial in the New York Morning News on 27 December 1845, which claimed the right of the United States to Oregon territory disputed by the British: "And that claim is by the right of our manifest destiny to overspread and to possess the whole of the continent which Providence has given us for the development of the great experiment of Liberty and federated self-government entrusted to us."
In his classic book Manifest Destiny: A Study of Nationalist Expansion in American History (1935), Albert K. Weinberg defines manifest destiny as being "in essence the doctrine that one nation has a preeminent social worth, a distinctively lofty mission, and consequently, unique rights in the application of moral principles." He relates how the idea soon became "a firmly established article of the national creed." The idea of manifest destiny is one of the clearest expressions of the belief in the exceptional nature of the United States. Territorial expansion was justified by Americans because they believed theirs was a special nation chosen by Providence to spread its virtues far and wide. As Weinberg has shown, nineteenth-century Americans laid claim to new land using a variety of justifications subsumed under the idea of manifest destiny. Americans believed they had a natural right to expand, and that territorial propinquity determined that they must spread across adjacent lands in North America. American expansion was also thought to be divinely blessed because it would cause the extension of democracy and freedom. Americans argued further that they should expand because they would use the land in ways more beneficial to and desirable for the progress of humankind than could its often sparsely distributed existing inhabitants.
The Mexican War saw a further element of manifest destiny emerge that became a central aspect of the belief in the exceptional nature of American interaction with other nations. Most Americans found moral vindication for the war in what Weinberg characterizes as "the conception of a religious duty to regenerate the unfortunate people of the enemy country by bringing them into the life-giving shrine of American democracy." The Mexican War seemed to confirm the belief among Americans that if the United States did involve itself in the affairs of other peoples, it was not in order to subjugate them but to help them realize their right to the same liberties and freedoms for which Americans had fought in the War of Independence. Countless Mexicans and Native Americans may have perceived events rather differently, but that was of little concern to the exceptional vanguards of civilization and progress.
By the mid-nineteenth century, then, with westward expansion justified by manifest destiny, the missionary strand of exceptionalism was becoming the dominant form. At the end of the nineteenth century, with historian Frederick Jackson Turner having declared the frontier closed, Americans undertook an overseas intervention which is generally regarded as the event that signaled the arrival of the United States as a world power. The highly popular Spanish-American War of 1898 was supposedly fought to secure basic rights of freedom for the oppressed peoples in Cuba and other Spanish colonies. Yet the war gave way to a conflict in the Philippines that left blood on American hands and ignited a nationwide debate over whether the United States was living up to its principles as an exceptional nation or whether it would assume an empire in the tradition of the Old World powers.