The Louisiana Purchase

• The United States expanded tremendously over the course of the 19th century. The first major westward expansion occurred in 1803 when President Thomas Jefferson negotiated the sale of the Louisiana Territory from France to the United States. The Louisiana Purchase, as the deal came to be known, allowed the United States to acquire 827,000 square miles for $15 million. With this one agreement, the United States doubled in land size.
  
  Out of the land that encompassed the Louisiana Purchase, part or all of the following U.S. states were formed: Louisiana, Missouri, Arkansas, Texas, Iowa, Minnesota, Kansas, Nebraska, Colorado, North Dakota, South Dakota, Montana, Wyoming, Oklahoma and New Mexico.
  
  

Expanding Through the Southwest

• Following the Louisiana Purchase in 1803, the United States' next major westward expansion occurred in 1845 and 1848 when the United States acquired most of the American Southwest. The first acquisition occurred in 1845 when the United States annexed the Republic of Texas and admitted it as a state.
  
  Three years after Texas became a state, the United States acquired the land that would eventually make up the remainder of the southwestern states. On February 2, 1848, the United States and Mexico signed the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War. Within this treaty was a clause that determined the formal territorial boundary between Mexico and the United States. The Mexican Cession, as it came to be known, granted the United States part or all of present-day Arizona, New Mexico, Utah, Colorado, Nevada and California.
  
  

Oregon Territory

• The final major westward expansion of the United States occurred in 1846 when the United States acquired Oregon Country from Britain. From 1818 until 1846, the United States and Britain jointly occupied the territory. In 1845, the two countries entered into negotiations to determine the formal boundary between the United States and Canada, and the main focus of these negotiations was the ownership of the Oregon Country. An agreement was reached in 1846, granting Oregon Country to the United States. The present-day states of Oregon, Washington and Idaho were created from Oregon Country.
  
  

Manifest Destiny and Slavery

• The story of the United States' westward expansion is directly tied to the idea of Manifest Destiny, which pervaded the country during the 19th century. This idea, rooted in religion, led to the belief that the United States was predestined to expand and control as much land on the North American continent as possible.
  
  Westward expansion in the United States also was directly related to the practice of slavery. While Manifest Destiny created a fervor regarding expansion, slavery created controversy. The acquisition of new territory became a competition between Northern abolitionists and Southern slave owners because every new Northern territory added was to be a nonslave state, while new Southern territories permitted slavery.