Manifest Destiny was a nationalistic belief holding that territorial expansion of the United States is not only inevitable but also divinely ordained.  The phrase was first coined by journalist John Louis O'Sullivan in an editorial supporting annexation of Texas in the July-August 1845 edition of the "United States Magazine and Democratic Review."  The phrase was used by expansionists to justify the acquisition of Texas from Mexico and later the acquiring of California, the Oregon Territory, and Alaska.  By the end of the 19th century the doctrine was being applied to the proposed annexation of various islands in the Caribbean Sea and the Pacific Ocean. 

Manifest Destiny (1845)
Author:  John L. O’Sullivan

It is time now for opposition to the Annexation of Texas to cease, all further agitation of the waters of bitterness and strife, at least in connexion with this question . . . It is time for the common duty of Patriotism to the Country to succeed;— if this claim will not be recognized, it is at least time for common sense to acquiesce with decent grace in the inevitable and the irrevocable.

Texas is now ours. Already, before these words are written, her Convention has undoubtedly ratified the acceptance, by her Congress, of our proffered invitation into the Union; and made the requisite changes in her already republican form of constitution to adopt it to its future federal relations. Her star and her stripe may already be said to have taken their place in the glorious blazon of our common nationality; and the sweep of our eagle's wing already includes within its circuit the wide extent of her fair and fertile land. . . .

Why, were other reasoning wanting, in favor of now elevating this question of the reception of Texas into the Union, out of the lower region of our past party dissensions, up to its proper level of a high and broad nationality, it surely is to be found, found abundantly, in the manner in which other nations have undertaken to intrude themselves into it, between us and the proper parties to the case, in a spirit of hostile interference against us, for the avowed object of thwarting our policy and hampering our power, limiting our greatness and checking the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions. This we have seen done by England, our old rival and enemy; and by France, strangely coupled with her against us. . . .

It is wholly untrue, and unjust to ourselves, the pretence that the Annexation has been a measure of spoliation, unrightful and unrighteous— military conquest under forms of peace and law— territorial aggrandizement at the expense of justice, and justice due by a double sanctity to the weak. This view of the question is wholly unfounded, and has been before so amply refuted in these pages, as well as in a thousand other modes, that we shall not again dwell upon it. The independence of Texas was complete and absolute. It was an independence, not only in fact but of right. No obligation of duty towards Mexico tended in the least degree to restrain our right to effect the desired recovery of the fair province once our own— motives of policy might have prompted a more deferential consideration of her feelings and her pride, as involved in the question. If Texas became peopled with an American population, it was by no contrivance of our government, but on the express invitation of that of Mexico herself; accompanied with such guaranties of State independence, and the maintenance of a federal system analogous to our own, as constituted a compact fully justifying the strongest measures of redress on the part of those afterwards deceived in this guaranty, and sought to be enslaved under the yoke imposed by its violation. She was released, rightfully and absolutely released, from all Mexican allegiance, or duty of cohesion to the Mexican political body, by the acts and fault of Mexico herself, and Mexico alone. There never was a clearer case. . . .

Nor is there any just foundation for the charge that Annexation is a great pro-slavery measure— to increase and perpetuate that institution. Slavery had nothing to do with it. Opinions were and are greatly divided, both at the North and South, as to the influence to be exerted by it on Slavery and the Slave States. That it will tend to facilitate and hasten the disappearance of Slavery from all the northern tier of the present Slave States, cannot surely admit of serious question. The greater value in Texas of the slave labor now employed in those States, must soon produce the effect of draining off that labor southwardly, by the same unvarying law that bids water descend the slope that invites it. Every new Slave State in Texas will make at least one Free State from among those in which that institution now exists— say nothing of those portions of Texas on which slavery cannot spring and grow— say nothing of the far more rapid growth of new States in the free West and Northwest, as these fine regions are overspread by the emigration fast flowing over them from Europe, as well as from the Northern and Eastern States of the Union as it exists. On the other hand, it is undeniably much gained for the cause of the eventual voluntary abolition of slavery, that it should have been thus drained off towards the only outlet which appeared to furnish much probability of it the ultimate disappearance of the negro race from our borders. The Spanish-Indian-American populations of Mexico, Central America and South America, afford the only receptacle capable of absorbing that race whenever we shall be prepared to slough it off— emancipate it from slavery, and (simultaneously necessary) to remove it from the midst of our own. Themselves already of mixed and confused blood, and free from the "prejudices" which among us so insuperably forbid the social amalgamation which can alone elevate the Negro race out of a virtually servile degradation even though legally free, the regions occupied by those populations must strongly attract the black race in that direction; and as soon as the destined hour of emancipation shall arrive, will relieve the question of one of its worst difficulties, if not absolutely the greatest.

. . . [T]here is a great deal of Annexation yet to take place, within the life of the present generation, along the whole line of our northern border. Texas has been absorbed into the Union in the inevitable fulfilment of the general law which is rolling our population westward, the connexion of which with that ratio of growth in population which is destined within a hundred years to swell our numbers to the enormous population of two hundred and fifty millions (if not more), is too evident to leave us in doubt of the manifest design of Providence in regard to the occupation of this continent. It was disintegrated from Mexico in the natural course of events, by a process perfectly legitimate on its own part, blameless on ours; and in which all the censures due to wrong, perfidy and folly, rest on Mexico alone. And possessed as it was by a population which was in truth but a colonial detachment from our own, and which was still bound by myriad ties of the very heart-strings to its old relations, domestic and political, their incorporation into the Union was not only inevitable, but the most natural, right and proper thing in the world— it is only astonishing that there should be any among ourselves to say it nay.