Saint Louis was once the site of significant Native American mounds built during the Mississippian period (see Mound Builders), but they were nearly all leveled as the city grew. In 1763 the location was chosen (1763) by Pierre LaClede for a fur-trading post. To honor Louis XV of France, it was named for his "name" saint, Louis IX of France. Transferred to the Spanish in 1770, it was retroceded to France in the time of Napoleon I and then sold to the United States along with the other lands of the Louisiana Purchase.
St. Louis, the gateway to the Missouri valley and the West, was the market and supply point for fur traders, mountain men, and explorers (including Lewis and Clark). The town grew rapidly after the War of 1812, when immigrants came in numbers to settle the West. St. Louis grew to be one of the greatest U.S. river ports; even after the railroads arrived in the 1850s, the river steamers remained extremely important.
The city was at the height of its population immediately following World War II. Between 1950 and 1990 the central city population decreased by half, and industry declined significantly in the same period. While many of the outlying suburbs grew steadily and developed industries, some, such as East Saint Louis, have been marked by high unemployment and poverty.