When John O'Sullivan used the term Manifest Destiny in 1845, he was specifically advocating for the annexation of Texas, which he said, was part of:
...our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.
The United States, with the election of James K. Polk, made manifest destiny its official policy. Texas's annexation later in 1845 set the stage for war with Mexico in 1846, a war that was fairly transparently about territorial acquisition. When the war ended with a US victory in 1848, Mexico ceded a vast tract of land that fulfilled American ambition to expand to the Pacific. These lands, acquired under the Treaty of Guadelupe Hidalgo, included almost the whole of the American Southwest, with the exception of a small tract of land in modern New Mexico and Arizona acquired by the US through the Gadsden Purchase a few years later.
In modern terms, the Mexican Cession included most or all of New Mexico, Arizona, Utah, California and Nevada. It also included those parts of modern day Wyoming and Colorado that had not been previously acquired in the Louisiana Purchase. The acquisition of these lands was a fulfillment of what many Americans had come to see as Manifest Destiny, but it also stoked the fires of sectional conflict as Americans debated whether the institution of slavery should be allowed in the new territories.