The mid-1840s were years of extraordinary territorial growth for the United States, a period in which the national domain increased by 1.2 million square miles, a gain of more than sixty percent. In 1845, the United States annexed Texas and the following year reached a settlement with Great Britain for control of the Pacific Northwest. Mexico's opposition to the annexation of Texas led to the Mexican War (1846–1848), which resulted in the U.S. acquisition of California and the American Southwest through the Treaty of Guadalupe Hidalgo. So rapid and dramatic was the process of territorial expansion that it came to be seen as an inexorable process, prompting many Americans to insist that their nation had a "manifest destiny" to dominate the continent.

Yet the expansionist agenda was never promoted by a clearly defined movement, nor did it enjoy broad bipartisan support. Some champions of Manifest Destiny favored...