In the early 1800s, Americans longed for lands in the American West. Western settlement focused on the Ohio River, parts of the Southeast and lands west of the Mississippi River. In essence, three methods for acquiring new lands existed: Native American treaties, purchase from foreign empires and war. These methods culminated in the broad concept of Manifest Destiny, or the idea that it was inevitable and morally acceptable for the United States to extend from the Atlantic to the Pacific Oceans.

Northwest Ordinance

By the late 18th century, settlers dreamed of lands west of the Appalachians and along the Ohio River. This area, northwest of the Ohio River and east of the Mississippi River, became known as the Northwest Territory. The federal government adopted the Northwest Ordinance in 1787 to administer the territory. Settlement precipitated a period of treaty-making and warfare with resident Native Americans. This conflict came to a head during the War of 1812, and many Native Americans in this region sided with the British during the war because the British were seeking trade partners not land. The course of the war worked in the favor of the Americans by ending disputes with Great Britain over control of the territory. It also caused a great deal of hardship to the Native Americans as settlers streamed west after the war and soldiers were given western land grants, which resulted in the removal of many tribes to the west of the Mississippi River.

Louisiana Purchase

Americans became very concerned about a Napoleonic France controlling the Mississippi River and curtailing economic activity in the West. In 1803, to Jefferson’s surprise, an overextended Napoleon actually offered the entire Louisiana Territory to the United States for about $15 million dollars. In a singular treaty with France, the United States nearly doubled its size. President Jefferson immediately sent Meriwether Lewis and William Clark on an expedition to the Missouri River and onward to the Pacific coast to explore the new territory. However, the Louisiana Purchase contained tens of thousands of Native Americans, who varied culturally, linguistically and in their friendliness to the United States. Many groups resisted American settlement, and it would be decades before the United States controlled these lands.

Manifest Destiny

In 1823, President Monroe issued a policy known as the Monroe Doctrine, which stated that the Western Hemisphere should be free from colonization by European empires. During Andrew Jackson’s administration in the 1830s, Americans acquired a renewed sense of nationalism, and in conjunction with the Monroe Doctrine, they came to believe that it was the United States' destiny to dominate the American continents and extend from coast to coast. In 1845, editor John O’Sullivan gave this idea a name, "Manifest Destiny." Others like Henry Clay, worried about how territorial acquisition would create more tension over the issue of slavery. Manifest Destiny and the extension of slavery immediately faced a test with the annexation of Texas in 1845.

Mexican-American War

Mexico did not formally agree to cede Texas to the United States, and it also disputed its western and southern borders. In addition, Americans eyed the promising territories of New Mexico and California. After failing to resolve the Texas border dispute and Mexico’s refusal to sell New Mexico and California for $30 million, President Polk and Congress declared war in May 1846. Many believed the Mexican-American War was simply a land grab, in essence a territorial conquest to extend the United States through the Southwest and on to California. For this reason, Americans were deeply divided about the necessity of this war. However, a successful war led to Mexico ceding these lands to the United States in 1848. Thus by the mid-1800s, the goals of the advocates of Manifest Destiny had largely been achieved.