Thanks to a high birth rate and brisk immigration, the U.S. population exploded in the first half of the 19th century, from around 5 million people in 1800 to more than 23 million by 1850.

Such rapid growth—as well as two economic depressions in 1819 and 1839—would drive millions of Americans westward in search of new land and new opportunities.

President Thomas Jefferson kicked off the country’s westward expansion in 1803 with the Louisiana Purchase, which at some 828,000 square miles nearly doubled the size of the United States and stretched from the Mississippi River to the Rocky Mountains. In addition to sponsoring the western expedition of Lewis and Clark of 1805-07, Jefferson also set his sights on Spanish Florida, a process that was finally concluded in 1819 under President James Monroe.

But critics of that treaty faulted Monroe and his secretary of state, John Quincy Adams, for yielding to Spain what they considered legitimate claims on Texas, where many Americans continued to settle.

In 1823, Monroe invoked Manifest Destiny when he spoke before Congress to warn European nations not to interfere with America’s Westward expansion, threatening that any attempt by Europeans to colonize the “American continents” would be seen as an act of war. This policy of an American sphere of influence and of non-intervention in European affairs became known as the “Monroe Doctrine.” After 1870, it would be used as a rationale for U.S. intervention in Latin America.

Cries for the “re-annexation” of Texas increased after Mexico, having won its independence from Spain, passed a law suspending U.S. immigration into Texas in 1830.

Nonetheless, there were still more Anglo settlers in Texas than Hispanic ones, and in 1836, after Texas won its own independence, its new leaders sought to join the United States. The administrations of both Andrew Jackson and Martin Van Buren resisted such calls, fearing both war with Mexico and opposition from Americans who believed calls for annexation were linked with the desire to expand slavery in the Southwest.

But John Tyler, who won the presidency in 1840, was determined to proceed with the annexation. An agreement concluded in April 1844 made Texas eligible for admission as a U.S. territory, and possibly later as one or more states.

Despite opposition to this agreement in Congress, the pro-annexation candidate James K. Polk won the 1844 election, and Tyler was able to push the bill through and sign it before he left office.

By the time Texas was admitted to the Union as a state in December 1845, the idea that the United States must inevitably expand westward all the way to the Pacific Ocean had taken firm hold among people from different regions, classes and political persuasions.

The phrase “Manifest Destiny,” which emerged as the best-known expression of this mindset, first appeared in an editorial published in the July-August 1845 issue of The Democratic Review.

In it, the writer criticized the opposition that still lingered against the annexation of Texas, urging national unity on behalf of “the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.”

As the phrase also appeared in a nearly identical context in a July 1845 article in the New York Morning News, its originator is believed to be John O’Sullivan, the editor of both the Democratic Review and the Morning News at the time. That December, another Morning News article mentioned “manifest destiny” in reference to the Oregon Territory, another new frontier over which the United States was eager to assert its dominion.

An 1842 treaty between Great Britain and the United States partially resolved the question of where to draw the Canadian border, but left open the question of the Oregon Territory, which stretched from the Pacific Coast to the Rocky Mountains over an area including what is now Oregon, Idaho, Washington State and most of British Columbia.

Polk, an ardent proponent of Manifest Destiny, had won election with the slogan “54˚ 40’ or fight!” (a reference to the potential northern boundary of Oregon as latitude 54˚ 40’) and called U.S. claims to Oregon “clear and unquestionable” in his inaugural address.

But as president, Polk wanted to get the issue resolved so the United States could move on to acquiring California from Mexico. In mid-1846, his administration agreed to a compromise whereby Oregon would be split along the 49th parallel, narrowly avoiding a crisis with Britain.

By the time the Oregon question was settled, the United States had entered into all-out war with Mexico, driven by the spirit of Manifest Destiny and territorial expansion.

The Treaty of Guadalupe Hidalgo, which ended the Mexican-American War in 1848, added an additional 525,000 square miles of U.S. territory, including all or parts of what is now California, Arizona, Colorado, New Mexico, Nevada, Utah and Wyoming.

Despite the lofty idealism of Manifest Destiny, the rapid territorial expansion over the first half of the 19th century resulted not only in war with Mexico, but in the dislocation and brutal mistreatment of Native American, Hispanic and other non-European occupants of the territories now being occupied by the United States.

U.S. expansion also fueled the growing debate over slavery, by raising the pressing question of whether new states being admitted to the Union would allow slavery or not—a conflict that would eventually lead to the Civil War.

Julius W. Pratt, “The Origin of ‘Manifest Destiny’,” The American Historical Review (July 1927).
Sean Wilentz, The Rise of American Democracy: Jefferson to Lincoln (New York: Norton, 2005).
Michael Golay, The Tide of Empire: America’s March to the Pacific
Era of U.S. Continental Expansion, History, Art & Archives: U.S House of Representatives.