Near the end of his active service, the U.S. invasion of Iraq began. Sheehan reenlisted, knowing that his unit would be sent there. Sheehan's division, the First Cavalry Division, was sent to Iraq. On March 19, 2004, Sheehan's Battery C, 1st Battalion, 82nd Field Artillery Regiment, arrived at FOB Camp War Eagle in Sadr City. On April 4, 2004, Sheehan was killed in action after volunteering to be part of a Quick Reaction Force to rescue American troops in the "Black Sunday" incident that began the Siege of Sadr City.