As a term, Manifest Destiny was first used by a magazine editor in 1845. In 1844, James K. Polk was elected to the presidency. He was the personification of Manifest Destiny, and he sought to acquire Oregon, Texas, and California; California was coveted for its minerals, fecund farmland, and great harbors. Polk was a determined and hard-working president, and his presidency was a great success for proponents of Manifest Destiny.
In a sense, Manifest Destiny predated Polk's presidency. The acquisition of Florida, the Louisiana Purchase, and America's unrelenting push westward were clear manifestations of what would later be known as Manifest Destiny. In this way, Manifest Destiny was a part of American history long before its culmination in the 1840s.
Ralph Waldo Emerson was prescient enough to realize that the acquisition of vast territories from Mexico would exacerbate sectional divisions over the slavery question. The South was eager to expand slavery to new territories in the West, and the North was equally determined to prevent its expansion.
There was bitter debate in the United States over the status of lands won from Mexico in 1848. Finally, the Compromise of 1850 was reached. This only delayed civil war for a decade, though.