Vernon County is located in the western region of the U.S. state of Missouri, on the border with Kansas. As of the 2020 census, the population was 19,707. Its county seat is Nevada. The county was organized on February 27, 1855, considerably later than counties in the eastern part of the state. It was named for Colonel Miles Vernon (1786–1867), a state senator and veteran of the Battle of New Orleans. This area was part of the large historic territory of the Osage Nation of Native Americans.

General Order No. 11 is the title of a Union Army directive issued during the American Civil War on August 25, 1863, forcing the abandonment of rural areas in four counties in western Missouri. The order, issued by Union General Thomas Ewing, Jr., affected all rural residents regardless of their allegiance. Those who could prove their loyalty to the Union were permitted to stay in the affected area, but had to leave their farms and move to communities near military outposts. Those who could not do so had to vacate the area altogether.