Manifest destiny was, by the late 1830s and early 1840s, an ideology for those who believed that the United States had the divinely ordained right to expand westward to the Pacific Ocean. By claiming that it was the right of the United States as a Christian, democratic country to annex territory in the Southwest, Manifest Destiny combined discourses of imperialism and freedom. Standing in the way of what they viewed as American progress was Mexico, which controlled most of what we know today as the American Southwest and California. It, along with the desire to spread slavery, was the driving ideological force in the debates over annexing Texas in the 1844 election, as well as the dispute with England over the Oregon Territory in the first year of Polk's presidency. When the United States annexed Texas in early 1845, the ultimate expression of manifest destiny, war between the US and Mexico became almost inevitable. The results of the war, including the addition of the entire Southwest, including California, were understood by men like Polk (though not to his Whig opponents like Henry Clay and Abraham Lincoln) to be a fulfillment of manifest destiny. Manifest Destiny, then, was the ideological force that contributed to the outbreak of the Mexican War.
Manifest Destiny was the belief that it was the United States's mission to rule over the entire North American continent. Texas had just joined the Union and the nation was anxious to expand. In 1844, the Democratic Party elected the dark horse candidate James K. Polk, who used a program of national expansion to defeat Henry Clay. At the treaty that ended Texas's war of independence, there was a border dispute--should Texas's southern border go to the Nueces River or should it extend to the Rio Grande? Texas chose the more southerly Rio Grande, while Mexico chose the Nueces. When Texas joined the Union, this issue still was not resolved. Polk, in order to force the issue, sent troops to the disputed territory. Mexican authorities fired on the troops, thus launching the war in which the United States took the entire Southwest, including California, from Mexico, only paying about fifteen million dollars as part of the treaty that ended the war.
Manifest Destiny was ideological, and many believed that it was ordained by God. These people thought that a higher power told democracy-loving Americans to take over the continent and use it for their own good. Of course, some Whigs in Congress argued loudly that this war was a conspiracy of slave power in America to gain more land for their own needs. Abraham Lincoln was one of these Whigs, and because of this he lost his seat in the House of Representatives. It was quite unpopular to say anything against Manifest Destiny, especially in Western states like Illinois.