Mexican War

Between 1846 and 1848 the United States successfully fought a war with Mexico that led Mexico to renounce all claims to Texas and cede its provinces of New Mexico and California to the United States. Many Americans rejoiced in the stunning victory. But some recognized that deep divisions over the status of slavery in New Mexico and California boded ill for their nation's future.

But the Democratic convention, where expansionist sentiment was stronger, denied Van Buren the nomination he coveted. Instead, the delegates chose James K. Polk of Tennessee, whose commitment to territorial expansion was clear and unqualified. To avoid the accusation of sectional favoritism, the Democratic platform cleverly united a demand for the admission of Texas with a demand for the acquisition “of the whole of the Territory of Oregon.” The platform also made the dubious assertion that the United States had a clear title to both. It followed, therefore, “that the re-occupation of Oregon and the re-annexation of Texas at the earliest practicable period are great American measures, which this convention recommends to the cordial support of the Democracy of the Union.” By combining the expansionist desires of South and West, the Democrats had found a winning formula. Throughout the campaign Manifest Destiny transcended all other issues, so much so that Clay began to shift his position on Texas. He would favor annexation after all if it could be accomplished without war and upon “just and fair terms.” But this commitment still sounded halfhearted when compared with the spread-eagle oratory and aggressive slogans of the Democrats.