The Bellevue War was a shootout between a posse led by Sheriff W. A. Warren and Thomas Cox against a group led by W. W. Brown that took place in Bellevue, Jackson County, Iowa Territory, on April 1, 1840. A large mural made by local artists commemorates the incident at the site of Brown's Hotel where it occurred. Political tensions in Bellevue dated back to 1837, when Yankee businessmen arrived at the town, causing tensions with the original settlers. The original settlers banded with wealthy hotelier W. W. Brown, who provided them with credit and an opportunity to work. A shooting on January 8, 1840, further intensified the situation.

Brown previously attempted to usurp territory representative Thomas Cox as the Democrats' candidate in the 1840 election. This infuriated Cox, who drafted a warrant for Brown's arrest following the shooting incident. Sheriff W. A. Warren, a former political opponent of Brown himself, authorized a posse to apprehend Brown. After an attempt to negotiate, the posse moved in on Brown's hotel. Brown apparently accidentally discharged his weapon, causing a gunfight to break out. Three more were killed on each side. The posse set the hotel on fire and captured those who attempted to escape. They were sentenced to receive a whipping and commanded to leave the county.

The first settlers in Bellevue, Iowa Territory, were miners from Galena, Illinois and southerners who distrusted eastern Yankees. In the spring of 1837, a large group of Yankees from Coldwater, Michigan, settled in Bellevue, creating tensions between them and the original settlers. One new arrival, William W. Brown, came with a substantial fortune and purchased the only hotel in town. The original settlers appreciated Brown's hospitality and open-handedness with credit. Brown would also pay high wages every winter to have the original settlers haul wood over the iced Mississippi River. Thus, the settlers formed a strong association with Brown.

Other Yankee settlers of Bellevue did not appreciate Brown's wealth. Brown was accused of acquiring wealth through dishonest methods. Some of the currency spent by Brown's associates was found to be counterfeit. Brown's associates in turn harassed those who accused Brown of wrongdoings. An increase in local cattle and horse rustlings made locals suspicious of Brown's group. In 1838, Brown announced his candidacy for Sheriff of Jackson County, where Bellevue was located. However, Whig William A. Warren of Burlington was able to secure the election. At the same election, Democrat Thomas Cox was elected as a representative to the first Iowa Territory general assembly. Brown was named a justice of the peace in 1839 and frequently defended his associates in court. The relationship at this point between Cox and Brown was considered cordial.

Cox's opinion of Brown quickly soured. Ebenezer Brigham, a colleague of Cox's from Illinois who was currently serving on the Wisconsin territorial council, came to Bellevue to search for a friend's stolen horses. Sheriff Warren recognized the description as a pair that had recently been apprehended from a boarder in Brown's hotel. Cox was infuriated at what he perceived to be Brown's role in the affair. In late 1839 or early 1840, the Democratic Party held a caucus to determine a candidate from Jackson County for the territorial house. Cox was busy platting Iowa City, so he was not present. However, he expected to be easily renominated. Instead, the caucus settled on Brown as their candidate. This began a rivalry between Cox and Brown. Cox ran as an independent, accusing Brown of associating with criminals. Cox won the election, causing further tensions in Bellevue as opponents of Brown rallied to his side.

Brown's associate James Thompson and Democrat James C. Mitchell got into a heated argument late in January 1840. Mitchell barricaded his house and put oak shutters on his windows to prevent Thompson from attacking. Mitchell also had a quarrel with one of his brothers. On January 8, the Democrats celebrated Jackson Day, which commemorated the Battle of New Orleans, and Mitchell was named the head of a celebratory ball. He explicitly forbid any allies of Brown from attending the affair, infuriating Brown's associates.

During the ball, Mitchell's brother left to go to James' house, ostensibly to retrieve clothing from a trunk. He claimed that the trunk belonged to his own wife and claimed to have taken a justice of the peace with him to prove that he was not going to take anything that did not belong to him. He was accompanied by Thompson, who had been drinking excessively. Upon their arrival, Thompson found a lady in a bed and attempted to assault her. The lady escaped and told James Mitchell about what happened. Mitchell armed himself and encountered Thompson. Both men raised their guns and attempted to shoot. Thompson's gun failed to properly fire, but Mitchell's fired a round into Thompson's chest, killing him. Mitchell returned to the ball, fearful of retribution from Brown's associates. Absalom Montgomery, who had followed Thompson in an attempt to persuade him away from combat, reported the incident to the sheriff. Sheriff Warren promised protection to Mitchell as long as he was willing to show in court.

The next morning, a coroner confirmed the cause of death. Mitchell was transferred to a justice office and charged with Thompson's murder. The case was scheduled to be heard at the next meeting of the circuit court. Mitchell was put in chains and left in a room in the justice office, as the county had no jail at the time. In the meantime, Cox helped to draft a warrant for the arrest of W. W. Brown and twenty-two associates for thieving, passing counterfeit money, robbing, and other charges. It was signed by Charles Harris, a justice of the peace living near Fulton. The Cox faction now had legal authority to arrest the Brown faction.