One week after the surrender of Confederate General Robert E. Lee at the Battle of Appomattox Court House, which ended all major engagements of the war, Cosgriff's unit moved towards Columbus, Georgia. Their intention was to capture the city's supply depots, naval yards, and weapons factories. On Easter Sunday, April 16, 1865, the 4th Iowa Cavalry and other regiments attacked a bridge over the Chattahoochee River which led into Columbus. When the Union general, James H. Wilson, called for cavalrymen to charge the covered bridge and capture the soldiers guarding it, Cosgriff volunteered. He and the other volunteers rode past two lines of entrenchments and fought hand-to-hand with the bridge guard. During the struggle, Cosgriff knocked down a Confederate flag bearer with the butt of his rifle, capturing both the flag and the bearer. The cavalrymen successfully subdued the bridge guard and captured two heavy guns. For these actions, Cosgriff was awarded the Medal of Honor two months later, on June 17. His official Medal of Honor citation reads: "Capture of flag in a personal encounter with its bearer." The flag, from an unknown regiment, is now held by the Museum of the Confederacy in Richmond, Virginia.