One could argue that the ideology of Manifest Destiny was important throughout this period, even before it was first articulated by the journalist John O' Sullivan in 1845. This was the notion that America was a unique, special country that had been entrusted by God with a mission to expand its territory, thus spreading liberty, democracy, and capitalism from coast to coast.
Up until the late 19th century Manifest Destiny was applied exclusively to the North American continent. The annexation of Texas was a crucial stage in its development, and was seen by the champions of Manifest Destiny as vital to the future economic growth of the United States, with its rapidly expanding population. Much the same justification was used for the acquisition of the Oregon territory from Great Britain—or most of it, at any rate—and of California from Mexico.
Yet even after the continental United States had taken the recognizable shape that it holds today, Manifest Destiny still retained a powerful hold on the American imagination. In the late 19th century, the ideology manifested itself in the Spanish-American War, in which the United States became an international superpower for the first time.
Although the McKinley Administration had no intention of colonizing Cuba and the Philippines in the way that Texas and California had been decades earlier, the United States' control over these formerly Spanish territories was nonetheless justified by reference to Manifest Destiny. In taking sovereign control of these territories, the United States, so it was argued, was spreading the values of democracy, liberty, and capitalism in much the same way as it had done by the previous expansion of domestic territory across the North American continent.