The support of unlimited expansion across the North American continent was most popularly referred to as being "Manifest Destiny." In this phrase, manifest refers to something as being obvious and clearly inevitable; destiny refers to fate, things that will happen in the future and the forces that will control the bringing about of those events.
The roots of the idea came to the New World with the first settlers in New England. The Reverend John Winthrop explained in one of his sermons that the Puritans had been selected by God and given divine blessings and encouragement to take control of and bring civilization to the wilderness to which they were going.
We shall find that the God of Israel is among us . . . when He shall make us a praise and glory that men shall say of succeeding plantations, the Lord make it like that of New England. For we must consider that we shall be as a city on a hill.
When President Thomas Jefferson signed the Louisiana Purchase, he doubled the size of the new country. In doing so, he proposed that northern European settlers should
cover the whole northern, if not the southern continent, with a people speaking the same language, governed in similar forms, and by similar laws
The actual phrase "manifest destiny" was first used by John L. O'Sullivan in 1845 when he wrote an article entitled "Annexation" for the United States Magazine and Democratic Review. As a part of this essay, he contended that it was the
manifest destiny of the United States to overspread the continent allotted by Providence for the free development of our yearly multiplying millions