Belleville is a city and the county seat of St. Clair County, Illinois, United States. It is located within Greater St. Louis. The population was 42,404 at the 2020 census, making it the most-populated city in Southern Illinois and in the Metro East region of Greater St. Louis. Due to its proximity to Scott Air Force Base, the population receives a boost from military and federal civilian personnel, defense contractors, and military retirees. It is also the seat of the Roman Catholic Diocese of Belleville and the National Shrine of Our Lady of the Snows.
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