When the Civil War began, Price was opposed to Missouri's secession. He was elected presiding officer of the Missouri State Convention on February 28, 1861, which voted against the state leaving the Union. The situation changed drastically, however, when Francis Preston Blair, Jr. and Capt. Nathaniel Lyon seized the state militia's Camp Jackson at St. Louis. Outraged by this, Price sided with the secessionists. Pro-Confederate Governor Claiborne Fox Jackson appointed him to command the newly reformed Missouri State Guard in May 1861. He led his young recruits (who nicknamed him "Old Pap") in a campaign to keep Missouri for the Confederacy. The most famous battle of the conflict was the Battle of Wilson's Creek. This occurred on August 10, 1861. The Missouri State Guard along with Confederate troops under the command of Brigadier General Benjamin McCulloch defeated Union troops in the Union Army of the West, under the command of General Lyon. Lyon was killed in the battle. After this victory, Price's troops launched an offensive into Northern Missouri, where they defeated Federal forces commanded by Colonel James Mulligan at the First Battle of Lexington. However, additional Union troops forced Price and Jackson's men to retreat, leaving most of the state to the Union.