John Alexander McClernand was an American lawyer and politician, and a Union general in the American Civil War. He was a prominent Democratic politician in Illinois and a member of the United States House of Representatives before the war. McClernand was firmly dedicated to the principles of Jacksonian democracy and supported the Compromise of 1850.
McClernand was commissioned a brigadier general of volunteers in 1861. His was a classic case of the politician-in-uniform coming into conflict with career Army officers, graduates of the United States Military Academy. He served as a subordinate commander under Ulysses S. Grant in the Western Theater, fighting in the battles of Belmont, Fort Donelson, and Shiloh in 1861–62.
A close friend and political ally of Abraham Lincoln, McClernand was given permission to recruit a force to conduct an operation against Vicksburg, Mississippi, which would rival the effort of Grant, his department commander. Grant was able to neutralize McClernand's independent effort after it conducted an expedition to win the Battle of Arkansas Post, and McClernand became the senior corps commander in Grant's army for the Vicksburg Campaign in 1863.