Fewer than 600 people lived in Hancock county, Illinois in 1839. They were farmers, mostly, and frontiersmen.
By the end of 1840, over 1,000 members of the Church of Jesus Christ of latter-day Saints had moved into the county, settling primarily in the newly founded city of Nauvoo. They came across the river as refugees.
By 1844, over 11,000 Mormons lived in Nauvoo and the surrounding area. The largest non-LDS centers of population were Warsaw and Carthage and the population of those two towns together totalled only about 1,000. Mormons outnumbered the old settlers by a margin of at least 10 to 1.
The old settlers were upset about all these foreigners just showing up and taking over. Many of them came from other countries; they talked funny, acted weird, and looked different. Financially, times were tough and the newcomers had no jobs and no skills. Most of them had worked in factories in the old country and there were no factory jobs to be had on the frontier, consequently they were often unemployed. If they did find jobs, they were willing to work for less pay than others, so they dragged down wages. Some of the Mormon newcomers lacked proper documentation, and many of them broke the law by trying to vote in elections, even though they weren’t citizens and couldn’t produce a birth certificate proof of citizenship on demand. They dragged down the economy of Hancock county, first by inflating real estate prices to the point that ordinary citizens couldn’t afford to buy anything, and then, when the bankruptcy laws were liberalized, their leader took advantage and immediately declared bankruptcy, thereby repudiating the debt on thousands of acres of land. Some of the newcomers also were convicted in a court of law of the crime of counterfeiting. It was clear that the newcomers were poor, unemployed, and prone to crime. And they cast their (often illegal) votes for the wrong candidates and parties.
Eventually the old settlers were able to use their connections in the legislature to get the Nauvoo charter repealed, and the newcomers found that their very existence in the county was a violation of the law.