On February 6, 1862, the Union Army under then Brigadier General Ulysses S. Grant captured Fort Henry, and ten days later, on February 16, they captured Fort Donelson. With these victories, his forces took about 12,000 to 15,000 Confederate prisoners. The army was unprepared to handle this many prisoners and scrambled to find places to house them. Cullum sent many prisoners to St. Louis before he received War Department instructions to direct 7,000 prisoners to Camp Douglas near Chicago. Cullum was chief engineer at the Siege of Corinth.