St. Louis is the largest city in the state of Missouri. It is located near the confluence of the Mississippi and the Missouri rivers. It was named after King Louis IX of France by the French traders and settlers in the 18th century. In 1804, the United States acquired St. Louis and Missouri as part of the Louisiana Purchase from France. Shortly afterward, the Lewis and Clark Expedition departed from St. Louis. Known as the Gateway to the West, it played an important part in the westward expansion of the United States.