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In page St. Louis:

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Settled by many Southerners in a slave state, the city was split in political sympathies and became polarized during the American Civil War. In 1861, 28 civilians were killed in a clash with Union troops. The war hurt St. Louis economically, due to the Union blockade of river traffic to the south on the Mississippi River. The St. Louis Arsenal constructed ironclads for the Union Navy.[citation needed]