President James Polk was a fervent believer in expansionism. He was directly responsible for adding a great deal of territory to the United States in the West, in the Oregon Territory and in New Mexico and California.
One of the beliefs that that President Polk embraced was the belief in Manifest Destiny. It was the belief that the United States was ordained and destined by God to expand across the North American Continent until reaching the Pacific Ocean.
It is the belief that the United States was meant to occupy the North American Continent from the Atlantic to the Pacific Ocean. It has at times wider interpretations that some felt should include all of Canada, Mexico, Central America and even Cuba.
For the most part however, most people of the era felt it meant expanding until reaching the Pacific Ocean. It was a term used by Andrew Jackson Democrats in the 1840s to promote the annexation of most of the Western United States.
President James Polk used the cry of “Manifest Destiny” during his 1844 election, calling for the annexation of “All of Oregon”, and “Fifty Four Forty or Fight”, both popular rallying cries supporting annexation.
President Polk was firm, and he began to terminate the joint occupation agreement regarding the Oregon territory, threatening to accelerate things up to possible War. This frightened the British, who finally compromised and signed the Oregon Treaty dividing the region along the 49th parallel in 1846.
James Polk used the Presidential bully pulpit and the cry of “Manifest Destiny” at different times, first with the British in getting them to sign the Oregon Treaty in 1846, and then a couple years later to force Mexico to settle in 1848 and compromise, selling California and New Mexico to the United States to help end the war between the US and Mexico.