The first native dwellers in Missouri were the Mound Builders, whose burial mounds can still be seen. At the time of the arrival of the first Europeans, several Indian tribes lived in the area, including the Osage, Missouri, Fox, and Sauk tribes. French explorers Marquette and Joliet arrived in 1673 and visited the juncture of the Mississippi and Missouri rivers. In 1682, La Salle claimed the entire Mississippi Valley, including Missouri, for France. The first white settlement was create by Jesuit missionaries around 1700. The first permanent settlement was Ste. Genevieve in 1735. St. Louis was founded in 1764. The French gave the territory to the Spanish in a secret treaty in 1762. Spain encouraged settlers, who flocked to the region. Daniel Boone was granted 800 acres of land and appointed a judge. In 1800, Napoleon forced the Spanish to return the area, which he then sold to the United States as part of the Louisiana Purchase in 1803. In 1812, Congress organized the Missouri Territory. Since Missouri had been settled largely by Southerners who owned slaves, its admission to the Union would have increased the number of slave states. The Missouri Compromise of 1820 created a pattern of adding a slave state for each free state. Missouri was admitted to the Union in 1821. At the time of statehood, Missouri represented the western frontier. Both the Oregon Trail, which carried pioneers west to Oregon, and the Sante Fe Trail to Sante Fe, New Mexico, originated at Independence, which became a prosperous community as a result. Bad feelings developed between slavery proponents in Missouri and opponents in Kansas, which continued into the Civil War. Sentiment within Missouri was split at the outbreak of the Civil War. The governor refused to send troops to assist the Union when called upon by President Lincoln in April 1861. Union troops quickly gained control of all of Missouri except a portion in the southeast and prevented Missouri from officially seceding. Harry S. Truman, a United States Senator from Missouri, was elected vice president along with Franklin Roosevelt in 1944 and succeeded Roosevelt as president on the latter's death in April 1945.