Abstract This chapter looks at the political, strategic, operational, and social dynamics of the war in Missouri in 1861. It describes and analyzes the events, personalities, and forces that led Brig. Gen. Nathaniel Lyon and his allies in Missouri to drive the pro-Southern government of Claiborne Fox Jackson from power, then considers the subsequent tenure in command of Maj. Gen. John C. Frémont and his efforts to deal with the aftermath of Jackson’s expulsion from Jefferson City, as well as the battlefield setbacks Union forces suffered at Wilson’s Creek and Lexington. All of this is considered in the context of the political, cultural, and social dynamics of a state that looked both south and west in defining its identity and considers how 1861 contributed to a reshaping of that identity.