Making his first annual address to Congress, President James K. Polk belligerently reasserts the 1823 Monroe Doctrine and calls for aggressive American expansion into the West. Polk's aggressive expansionist program created the outline of the modern American nation.
The Monroe Doctrine was the creation of Polk's predecessor, James Monroe, who argued that all European influence should be removed from the neighborhood of the United States for reasons of national security. As a result, throughout the first half of the 19th century, Americans had worked to undermine European claims on the continent, often by peacefully annexing European territories.
Polk's extension of the Monroe Doctrine, however, carried a far more aggressive agenda, which reflected his willingness to use force to create a nation stretching across the continent. Polk felt that such expansion was part of America's "manifest destiny." Polk's vision of America's future included making the Rio Grande River the southern border of Texas, the acquisition of California, and an end to sharing control of Oregon territory with the British. Always slightly paranoid about the Europeans, Polk worried that France would insist on maintaining a balance of power in North America and that Great Britain would try to keep the U.S. from acquiring Texas and California. In fact, neither nation was very aggressive about resisting American expansionism, and Great Britain peacefully surrendered its claim to the Oregon territory south of the 49th parallel in 1846.
Polk's ambition to take the Mexican-controlled California and the rest of the Southwest away from Mexico proved more difficult to realize. Six months after his speech to Congress, Polk's decision to send troops to the Rio Grande in Texas led to war with Mexico. Despite Polk's fears, neither France nor Great Britain leapt to the aid of the Mexicans in the war, leaving the U.S. free to act as it wished. When the Americans emerged victorious in 1848, the Treaty of Guadalupe Hidalgo gave Polk precisely what he wanted: the vast northern provinces of the Mexican empire that would one day become the states of Texas, California, Arizona, New Mexico, Nevada, and Utah. This land was the final piece of the puzzle needed to create the territory of today's United States.