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Westward Expansion (1790-1861)

This map is part of a series of 17 animated maps showing the history of The United States: a territorial history


Searching for new lands and protection from foreign invasion, the young nation quickly extended its external borders.

In 1803, Jefferson negotiated the Louisiana Purchase with France. This covered the area around New Orleans that was already inhabited and vast territories, largely unexplored, to the west of the Mississippi. With this acquisition, the United States doubled in size and staked its claim to the rest of the continent.

A treaty between the US and Great Britain, signed in 1818, established the 49th parallel as the frontier between their territories as far as the Rocky Mountains and provided for joint control of the Oregon Territory.    

In the South, the United States annexed the coastal area to the east of the Mississippi and entered into negotiations with Spain for the purchase of Florida.

The Adams-Onis Treaty marked out the frontier between the Spanish and American possessions from the Gulf of Mexico to the Pacific Ocean.

In 1821, Mexico’s independence marked the end of the Spanish presence, soon after that of the French, in North America.

In 1823, President Monroe declared the American continent off-limits to the European Powers.

Meanwhile the pioneers advanced gradually across the continent, encroaching more and more on Indian territory. In 1830, Congress voted a law authorizing the deportation of Indian tribes to ‘reserves’ west of the Mississippi.

Towards the mid 19th century, the ideology of ‘Manifest Destiny’ provided a justification for further territorial expansion:

- in the north, with the end of border disputes between Maine and New Brunswick, an agreement was reached with the United Kingdom in 1846 on the division of the Oregon Territory along the 49th parallel.

-in the south, Texas was brought into the Union. The Guadalupe Hidalgo Treaty marked the end of the Mexican War and led to the integration of huge territories in New Mexico and California.

In 1853, a last minor acquisition, the Gadsden Purchase, brought the southern part of New Mexico into the Union.

In parallel with the establishment of new international borders, statehood was granted to a growing number of territories.

In 1790, the Union consisted of 13 states, increasing to 24 by 1821, and 34 in 1861.

The Union strived to maintain a delicate balance between free and slave states, but the admission of California led to rising tension between the defenders and opponents of slavery in the 1850s.

This led to the secession of 11 Southern States from the Union and the outbreak of the Civil War.

The first act of war was the bombardment of Fort Sumter in Southern Carolina, launching four long years of bloody war between the Union States and the Confederate States.