Warsaw’s greatest claim to fame is due to the role it played as a center of virulent opposition to the Mormons who had settled in Nauvoo, 17 miles to the north. (The Mormons had originally tried to settle just south of Warsaw but there was so much opposition from Warsaw that they gave up on that plan). The Warsaw Signal newspaper, published in Warsaw, led a media campaign against the Mormon settlement in Nauvoo. Indeed, the new religion and its adherents were denounced. The agitation in Warsaw was the proximal cause of the murders of the Mormon leader, Joseph Smith, and his brother Hyrum, in 1844, while they were being held in the nearby Carthage jail. Five of nine men who murdered the Smiths in Carthage were from Warsaw. The house where the newspaper lived still exists (and is occupied) at the end of Main Street before the descent to the Mississippi River. A guide to places of interest in Warsaw during the Mormon conflict can be found here.

An interesting historical interpretation of Warsaw and the Mormons has been advanced by Brian J. Stutzman in his book The History of Warsaw Illinois. Including the Mormon Period, 2nd edition (2018). A summary of the book with an embedded video is available here:
https://wheatandtares.org/2019/08/12/anti-mormon-triangle-carthage-nauvoo-warsaw/
and: https://www.youtube.com/watch?v=xznbuBCfGaA