The watershed and mouth of the Missouri-Mississippi River system became known as Louisiana and was from the earliest days of discovery considered strategically important by many European nations. As a result, it had an unusually wide array of ethnic influences during the 17th and 18th centuries. The acquisition of Louisiana by the United States in 1803 ensured control of the continental interior and provided ample land for migrant and immigrant farmers of the new republic.
The first European to explore the region was the Spanish explorer Hernando de Soto (1541–42). Finding no mineral wealth, the Spanish paid little attention to the region. In 1682, René-Robert Cavelier de La Salle claimed the Mississippi River Valley for France, naming it Louisiana in honor of King Louis XIV. Attempts by Scottish investor John Law to colonize the region failed between 1717 and 1720, with the exception of New Orleans, founded in 1718. Disappointed with the little income generated from the region, France ceded Louisiana to Spain from 1762 to 1800, though it was done secretly and Spain did not gain firm control until 1769. During the 1790s, the sugar industry was established, and the colony began to flourish. In addition to Spanish settlers, an increasing number of Europeans from other countries began to settle in the region. About 4,000 French colonists from Acadia migrated to Louisiana following the capitulation of Montreal in 1760; the descendants of these migrants became known as Cajuns. During the American Revolution (1775–83), a significant number of Americans also came to New Orleans, which had been granted to the Continental Congress as a base of operations during the conflict.
In 1800, Napoleon coerced Spain into ceding Louisiana back to France, though a full transfer of control was never effected. In December 1803, in a transaction known as the Louisiana Purchase, France sold the entire Mississippi valley to the United States for about $15 million. The huge region was subdivided during the 19th century, with the present-day state of Louisiana being admitted to the Union in 1812. With the advent of the steamboat, a great period of commercial expansion began after 1812, and thousands of settlers arrived between the end of the War of 1812 (1815) and the beginning of the Civil War (1860). The northern territories of Louisiana eventually became all or part of the states of Arkansas, Missouri, Iowa, Minnesota, North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Texas, New Mexico, Colorado, Wyoming, and Montana and home to thousands of European immigrants seeking land following the Civil War, which ended in 1865.