By the election of 1844, westward expansion in the United States was well on its way, and the goal of creating a bicoastal nation was nearly complete. Although newspaper editor John O'Sullivan is credited with coining the phrase "manifest destiny" to describe the philosophy of territorial acquisition, the push westward began when the colonists first settled on the American east coast. James Polk embraced the idea and openly promoted manifest destiny as a platform in his election campaign in 1844 and as president of the United States.
Many historians view James Polk as the last of the Jacksonian-era presidents. Jackson and Polk were close allies in the Tennessee legislature, and Polk copied much of Jackson's rhetorical populist message and electioneering style. Polk's election campaign slogan during the election of 1844 was "54'-40' or fight," a reference to his intention to acquire the Oregon Territory if elected. Polk's brash campaign claim to take the Oregon Territory in place of having a sharing arrangement with Britain stood in contrast to his opponents during the election, who shied away from taking a position, thinking it too controversial. While Oregon was controversial, Polk's view on annexing Texas may have been even more so.
Since 1836, James Polk had been clear in publicly supporting the annexation of the territories in Texas. Texas had seceded from Mexico even though the Mexican and American governments were hotly disputing whether Texas could formally secede without consent from the Mexican government. By the election of 1844, tensions in the territory had erupted into several conflicts. During the campaign, Polk made clear his intentions to annex Texas as a state.
Once he was elected, Polk carried through on his campaign promises. In 1846, Polk negotiated a treaty with Britain to establish a border for the Oregon Territories, with the United States acquiring all of the territories south of the forty-ninth parallel. The same year, the United States went to war with Mexico under the pretense of protecting Texans from Mexican soldiers' attacks. Two years later, the war ended with an Americans victory and Texas's annexation into the United States. In addition to Texas, the United States added nearly 500,000 square miles of new territory extending west from the Rio Grande to the Pacific Ocean, including California.