Colonel Marsh and the 20th were stationed at Bird's Point, Missouri until joining Ulysses S. Grant's expedition to Fort Henry as a part of W. H. L. Wallace's 2nd Brigade in John A. McClernand's 1st Division. At the battle of Fort Donelson Marsh was positioned on the right of the Union lines. McClernand's entire division was hit hard during the Confederate breakout attempt on February 15. Despite being hard pressed Marsh managed a counterattack which momentarily checked Simon B. Buckner's division and saved an Illinois artillery battery from capture.