The Camp Jackson affair, also known as the Camp Jackson massacre, occurred during the American Civil War on May 10, 1861, when a volunteer Union Army regiment captured a unit of secessionists at Camp Jackson, outside the city of St. Louis, in the divided slave state of Missouri.

St. Louis is an independent city in the U.S. state of Missouri. It is located near the confluence of the Mississippi and the Missouri rivers. In 2020, the city proper had a population of 301,578, while its bi-state metropolitan area, which extends into Illinois, had an estimated population of over 2.8 million. It is the largest metropolitan area in Missouri and the second largest in Illinois. The city's combined statistical area (CSA) is the 20th largest in the United States.