The Great American Desert and Manifest Destiny
A Continent Mostly Unknown
In 1840 the United States stretched from the Atlantic to the Mississippi, but most Americans had never set foot west of that great river. Beyond it lay a continent few easterners could picture clearly. The land rose from rolling prairies to the vast, treeless Great Plains, climbed sharply into the Rocky Mountains, and finally fell away through deserts and mountain ranges to a fertile Pacific coast. Rivers were few; rainfall west of the 100th meridian was unreliable; and timber, the staple building material of the East, ran out somewhere around the Missouri.
In 1820 the army explorer Stephen Long crossed the Plains and produced an influential map. He stamped the words ‘Great American Desert’ across the heart of the continent and concluded the region was “almost wholly unfit for cultivation, and of course uninhabitable by a people depending upon agriculture”. For the next twenty years schoolbooks repeated the verdict. The Plains were a barrier to be crossed, not a place to live.
Stephen Long’s 1820 expedition labelled the Plains the ‘Great American Desert’ on US maps. The verdict shaped policy until the 1840s, when settlers began to see the same land as a route to the fertile West Coast rather than a barrier.
Why the ‘Desert’ Idea Began to Crack
By the 1840s the East was changing fast. Cities were crowded with immigrants, the 1837 financial panic had thrown thousands of farmers off their land, and soils in New England and the Atlantic states were exhausted after generations of cropping. Pull factors became powerful: missionaries returning from Oregon described its rich valleys; California offered a Mediterranean climate and good harbours; and gold strikes after 1848 made the journey suddenly worth the risk. The Plains were no longer a destination but a gateway.
Coining Manifest Destiny
In July 1845 the editor John O’Sullivan published an article in the New York magazine the Democratic Review claiming it was “our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions”. The phrase caught on instantly. It bundled together three older ideas Americans had held for decades: a religious sense that God favoured the United States; a racial belief in the superiority of Anglo-Saxonism; and a political confidence that republican institutions were the best in the world.
Importantly, ‘manifest’ meant obvious or beyond argument, and ‘destiny’ meant divinely fixed. Together the words declared that westward expansion was not a choice; it was a duty. Anyone who stood in the way — Mexicans south of the border, British settlers in Oregon, the Plains nations — was, on this logic, opposing the will of Providence itself.
Push factors drove people away from the East: overcrowded cities, the 1837 Panic, soil exhaustion, and (for groups like the Mormons) religious persecution. Pull factors drew them west: cheap or free land in Oregon and California, gold strikes after 1848, missionary reports of fertile valleys, and the promise of a fresh start. Manifest Destiny linked the two by giving migration a moral meaning — settlers were not running from failure; they were carrying American civilisation across the continent.
From Idea to Policy
Manifest Destiny was a slogan, but it shaped concrete decisions almost at once. President James K. Polk, elected in 1844, came to office promising to acquire Texas, Oregon and California. The 1845 annexation of Texas sparked the Mexican-American War (1846–48). Victory and the Treaty of Guadalupe Hidalgo (1848) added a vast territory — all of present-day California, Nevada and Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming. The Louisiana Purchase of 1803 had already doubled the country; Polk added more than a million square miles more in under five years.
Manifest Destiny also shaped attitudes towards the people already living on the land. The doctrine treated North America as if it were empty waiting room, but the Plains were home to hundreds of thousands of Native people in dozens of nations. The historian Patricia Nelson Limerick has argued that Manifest Destiny’s real role was not to describe a vacuum but to legitimise dispossession — to make the removal and confinement of those nations look natural and good. That long shadow fell over every later policy in this unit.
One reading, dominant for most of the 20th century, presented westward expansion as the engine of American democracy — a frontier that toughened settlers and produced free institutions. A second, revisionist reading from the late 1980s onwards reframed the same story as a process of conquest, in which religion and race were used to justify taking land from people already on it. The two readings often appear in interpretation questions: top answers explain why each writer reaches a different verdict, not just what each one says.
Manifest Destiny had three major consequences in the 1840s alone: war with Mexico (1846–48); the gain of vast new territory through the Treaty of Guadalupe Hidalgo (1848); and a sharper, more confident push of settlers and policy onto land already occupied by Native nations.