St. Louis is an independent city and major U.S. port in the state of Missouri, built along the western bank of the Mississippi River, which marks Missouri's border with Illinois. The city had an estimated 2018 population of 308,626, and is the cultural and economic center of the Greater St. Louis area (home to 2,807,338 people), making it the largest metropolitan area in Missouri and the 21st-largest in the United States. Prior to European settlement, the area was a major regional center of Native American Mississippian culture. The city of St. Louis was founded in 1764 by French fur traders Pierre Laclède and Auguste Chouteau, and named after Louis IX of France. In 1764, following France's defeat in the Seven Years' War, the area was ceded to Spain and retroceded back to France in 1800. In 1803, the United States acquired the territory as part of the Louisiana Purchase. During the 19th century, St. Louis developed as a major port on the Mississippi River. In the 1870 Census, St. Louis was ranked as the 4th-largest city in the United States. It separated from St. Louis County in 1877, becoming an independent city and limiting its own political boundaries. In 1904, it hosted the Louisiana Purchase Exposition and the Summer Olympics. The economy of metropolitan St. Louis relies on service, manufacturing, trade, transportation of goods, and tourism. Its metro area is home to major corporations, including Anheuser-Busch, Express Scripts, Centene, Boeing Defense, Emerson, Energizer, Panera, Enterprise, Peabody Energy, Ameren, Post Holdings, Monsanto, Edward Jones, Go Jet, Purina and Sigma-Aldrich. Nine of the ten Fortune 500 companies based in Missouri are located within the St. Louis metropolitan area. This city has also become known for its growing medical, pharmaceutical and research presence. St. Louis has 2 professional sports teams: the St. Louis Cardinals of Major League Baseball and the St. Louis Blues of the National Hockey League. The city is commonly identified with the 630-foot (192 m) tall Gateway Arch in the city's downtown.