Final answer:
St. Louis began as a vital trade hub within the Missouri Territory, established after the Louisiana Purchase. The city was entwined in the controversial debate over Missouri's status as a slave or free state when Congress began discussions in 1817.
Step-by-step explanation:
St. Louis was established as part of the Missouri Territory, a significant expansion of the United States following the Louisiana Purchase. The city emerged as a prominent Mississippi River town, becoming an essential trade hub for the northern Mississippi Valley and the Greater West. The debate over whether Missouri, including St. Louis, would enter the Union as a free or slave state ignited profound sectional controversy when Congress opened its debate in 1817. St. Louis started out as a bustling trade center and was deeply tied to the economic and political dynamics of slavery and statehood during the early 19th-century expansion of the United States.