History: Wild West By Timothy H. O'Sullivan

After being honorably discharged, he rejoined Brady's team. In July 1862, O'Sullivan followed the campaign of Maj. Gen. John Pope's Northern Virginia Campaign. By joining Alexander Gardner's studio, he had his forty-four photographs published in the first Civil War photographs collection, Gardner's Photographic Sketch Book of the War. In July 1863, he created his most famous photograph, "The Harvest of Death," depicting dead soldiers from the Battle of Gettysburg. He took many other photographs documenting the battle, including "Dead Confederate sharpshooter at foot of Little Round Top", "Field where General Reynolds fell", "View in wheatfield opposite our extreme left", "Confederate dead gathered for burial at the southwestern edge of the Rose woods", "Bodies of Federal soldiers near the McPherson woods", "Slaughter pen", and others.
In 1864, following Gen. Ulysses S. Grant's trail, he photographed the Siege of Petersburg before briefly heading to North Carolina to document the siege of Fort Fisher. That brought him to the Appomattox Court House, the site of Robert E. Lee's surrender in April 1865.
From 1867 to 1869, he was official photographer on the United States Geological Exploration of the Fortieth Parallel under Clarence King. The expedition began at Virginia City, Nevada, where he photographed the mines, and worked eastward. His job was to photograph the West to attract settlers. O'Sullivan's pictures were among the first to record the prehistoric ruins, Navajo weavers, and pueblo villages of the Southwest. In contrast to the Asian and Eastern landscape fronts, the subject matter he focused on was a new concept. It involved taking pictures of nature as an untamed, pre-industrialized land without the use of landscape painting conventions. O'Sullivan combined science and art, making exact records of extraordinary beauty.