Peter Joseph Osterhaus (1823-1917) was a German-American soldier, born in Koblenz, Rhenish Prussia. After serving for some time as a Prussian army officer, he emigrated to the United States and settled in St. Louis. At the outbreak of the Civil War he was appointed a major of the Second Missouri Volunteers and during the first year of the war was employed in Missouri and Arkansas, where he took a conspicuous part in the battles of Wilson's Creek (August 10, 1861) and Pea Ridge (March 7-8, 1862). At the latter he commanded the troops that first made contact with Confederate forcers advancing on the Union left. In 1863 he commanded a division in the Battle of Port Gibson, where he displayed tactical ability in prying Confederate defenders out of a favorable position. Osterhaus continued in division command during the Battle of Champion's Hill and at the Battle of Big Black River Bridge, where he was slightly wounded. Osterhaus' division made an unsuccessful first attack on the defenses of Vicksburg, the first act of the Siege of Vicksburg. His division helped cover the siege against intervention by the Confederate forces of Joseph Johnston, and he took part in William T. Sherman's advance on Jackson, Mississippi that was designed to protect the rear of the Army of the Tennessee in its siege operations.
After the fall of Vicksburg, Osterhaus' division was transferred to Tennessee. In the Chattanooga (November 23 to November 25) he aided General Hooker in the capture of Lookout Mountain. He was then assigned to General Sherman's army, and after the capture of Atlanta received command of the Fifteenth Corps, one of the four corps into which the army was consolidated, in the March to the Sea. He was mustered out of the service on January 15, 1866, and the same year was appointed United States Consul at Lyons, France, but subsequently made his home in Germany, at Duisburg. He retired from the army in 1905, and was in 1915 the oldest pensioner on the army list.