upon the outbreak of the american civil war, he raised the "mcclernand brigade" in illinois, and was commissioned brigadier general of volunteers on may 17, 1861. his commission as a general was based not on his brief service in the blackhawk war, but on lincoln's desire to retain political connections with the democrats of southern illinois. he eventually resigned his congressional seat effective october 28. he was second in command under ulysses s. grant at the battle of belmont in missouri on november 7, 1861, and commanded the 1st division of grant's army at fort donelson; his division, whose flank was not properly anchored on an obstacle, was struck by a surprise attack on february 15, 1862, and driven back almost two miles before he was able to get reinforcements. on march 21, he was promoted to major general of volunteers for his service at fort donelson. at the battle of shiloh on april 6–7 he commanded a division of the army of the tennessee, which resisted, along with that of william tecumseh sherman, the strong confederate assaults around shiloh church.