The United States had been expanding ever since the close of the Revolutionary War, moving westward as its population increased from 5.3 million in 1800 to 23.2 million in 1850. These expansionist ambitions -- fueled by a need for fertile farmland, new economic opportunities and a sense of manifest destiny -- reached full fruition during the 1840s, the decade during which the United States would see its fastest territorial growth.

Seeking New Territory to Settle

The United States gained vast tracts of territory in the west in the Louisiana Purchase of 1803 and received Florida and the Gulf Coast east of the Mississippi from Spain in the Adams-Otis Treaty of 1819. But by the 1830s, American settlers were clamoring for land in Mexican-owned Texas, pushing ahead of their government into these territories. After battling Mexico, Anglo settlers in Texas won independence in 1836, but tensions remained high. A similar situation occurred during the 1830s in Oregon Territory, which was claimed by both the United States and Great Britain. As American settlers began pouring into the fertile Willamette Valley, Britain and the United States sought for a negotiated solution to the problem.

Expansion Under Polk

In 1844, James Polk, one of America’s most aggressively expansionist presidents, was elected. During the course of his four years as chief executive, he set most of the United States’ present borders as a nation. In 1845, he agreed to a treaty with Great Britain that provided the U.S. a northern boundary at the 49th parallel and gave America present day Oregon, Washington and Idaho. In 1846, he went to war with Mexico and ended up with a stunning victory in which Mexico ceded the U.S. the present-day states of Arizona, Utah, Nevada, California, much of New Mexico and portions of Wyoming and Colorado. American territory was increased by one-quarter. A further treaty with New Grenada provided the U.S. with future right of way over the Isthmus of Panama, preparing the way for the construction of the Panama Canal in the next century.

Imperial Maritime Ambitions

The journalist John L. O’Sullivan coined the term “manifest destiny” in 1845, meaning that it was the God-given destiny of America to control all the land within the American continent. After Polk’s territorial acquisitions, Americans flocked to the new lands opened up, heading in large numbers for California during the Gold Rush of 1849. It wasn’t only land that Americans sought, but the business opportunities that would be found in the Californian ports, for American imperialism in the 1840s also took the form of maritime expansion in the Pacific. The U.S. signed a treaty with China in 1844 that opened up diplomatic relations and trade possibilities between the two countries, for which America needed Pacific ports.

The Beginnings of World Power

Since American merchants needed to make stopovers during the course of the long journey from California to China, the United States became a presence in Pacific islands like Fiji, Samoa, Hawaii and the Marshall Islands, establishing itself as a nascent Pacific power. Within sixty years, America would be a major world power in the Pacific with important territorial holdings and commercial interests.