Pro-Confederate Governor Claiborne Fox Jackson appointed him to command the newly reformed Missouri State Guard in May 1861, and Price led his recruits (who nicknamed him "Old Pap") in a campaign to expel Lyon's troops. By then Lyon had seized the state capital and pushed through a bill to remove Governor Jackson from office and replace him with an unelected Union governor. The climax of the conflict was the Battle of Wilson's Creek, fought on August 10, where Price's Missouri State Guard, supported by Confederate troops led by Brigadier General Benjamin McCulloch, soundly defeated Lyon's Army of the West; the Union general was killed during the battle. Price's troops launched an offensive into northern Missouri, defeating the Federal forces of Colonel James Mulligan at the First Battle of Lexington. However, the Union Army soon sent reinforcements to Missouri, and forced Price's men and Governor Jackson to fall back to the Arkansas border. The Union retained control of most of Missouri for the remainder of the war, although there were frequent guerrilla raids in the western sections.