Manifest Destiny was the belief during the mid nineteenth century that it was God's will that the United States of America should overspread the entire North American continent. It was first expressed in a newspaper article by John L. O'Sullivan who wrote:
Our manifest destiny is to overspread the continent allotted by Providence fo the free development of our yearly multiplying millions.
Although it did not become official government policy, Manifest Destiny played prominently in American foreign and domestic affairs. It was largely behind several attempts to secure Canada as part of the United States; was also a factor in the annexation of Texas into the Union, and was a prominent factor in the Mexican War of 1848. Mexico had broken diplomatic relations with the U.S. over the annexation of Texas; and was further offended when President James K. Polk offered to buy California. When the U.S. managed to provoke a confrontation between the Neuces and Rio Grande rivers, President Polk used the occasion to deliver a war message to Congress. In the ensuing war, the U.S. not only defeated Mexico, but also received large portions of the Southwest, including California, Arizona and New Mexico. Only a small portion of the present lower 48 states remained in Mexican hands. This was purchased as part of the Gadsden Purchase of 1853; at which point Manifest Destiny as Americans understood it had been fulfilled.
Manifest Destiny was the belief that the U.S. was destined to secure territory from "sea to sea," from the Atlantic to the Pacific Ocean. The phrase was coined by New York journalist John L. O'Sullivan.
Manifest Destiny was the 19th Century American belief that the United States were destined to expand across the continent. It was used by the Democrats in the 1840's to justify the war with Mexico. Manifest Destiny played its most important role in the Oregon boundary dispute with Britian. It had serious consequences for the Native Americans, since continental expansion meant the occupation and annexation of Native American's land, which sometimes expanded to slavery. After the turn of the 19th century to the 20th, the phrase Manifest Destiny declined in usage as territorial expansion ceased to be promoted as part of America's destiny.