President Polk's vision of a country that stretched from the Atlantic to the Pacific was not a new idea, but soon after his election, Americans received a well-phrased rationale to justify expansion. In 1845, John L. O'Sullivan, publisher of the Democratic Review, wrote that it was the nation's "manifest destiny to overspread and to possess the whole of the continent which Providence has given us for the development of the great experiment of liberty and federated self-government entrusted to us."
What's the big deal about Manifest Destiny?
The two words "manifest destiny" quickly caught on, soon coming to mean that those who favored expansion had God on their side and were engaged in the noble task of spreading democracy. Despite the fact that the expansionist doctrine was based partly on the notion of racial superiority -- O'Sullivan referred to the "superior vigor of the Anglo-Saxon race" -- it appealed both to supporters of slavery, who wanted Texas annexed, and to antislavery advocates, who favored adding California and Oregon to the Union.
Proponents of manifest destiny claimed that a continental United States would benefit from trade with Asia, from the commercial advantages of San Francisco Bay and Puget Sound, and from lower tariffs. Sea-to-sea expansion would also safeguard democracy, give the nation room to grow, and preserve the essential character of the country as an agricultural nation in the Jeffersonian tradition.