Leavenworth is the largest city in and the county seat of Leavenworth County, Kansas, United States. As of the2010 census, the city population was 35,251.
Leavenworth, founded in 1854, was the first city incorporated in the newly formed state of Kansas. The city developed south of Fort Leavenworth, which was established as Cantonment Leavenworth in 1827 by ColonelHenry Leavenworth. Its location on the Missouri River made it a destination for refugee African-American slaves seeking freedom from the slave state of Missouri in the antebellum years. Abolition supporters helped them find refuge. In the years before the American Civil War, Leavenworth was a hotbed of anti-slavery and pro-slavery agitation, often leading to open physical confrontations on the street and in public meetings.On April 3, 1858, it was in Leavenworth, Kansas that the "Leavenworth Constitution" for the state of Kansas was adopted. Although the federal government never approved this early version of the state constitution, it was considered one of the most radical of the four constitutions drafted for the new territory because it included freed blacks as citizens.Fort Leavenworth was located outside the city limits until its territory was annexed by the city on April 12, 1977.