At the outbreak of the Civil War Osterhaus was appointed a
major of the
2nd Missouri Infantry Regiment and during the first year of the war was employed in
Missouri and
Arkansas, where he took a conspicuous part in the
Battle of Wilson's Creek and
Battle of Pea Ridge. At Pea Ridge he commanded the troops that first made contact with
Confederate forces advancing on the Union left. He was promoted to
brigadier general on June 9, 1862. 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. at
Vicksburg National Military Park Osterhaus continued in division command during the
Vicksburg Campaign, fighting in the
Battle of Champion Hill and at the
Battle of Big Black River Bridge, where he was slightly wounded. Osterhaus's 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 Gen.
Joseph E. Johnston, and he took part in Maj. Gen.
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's division was transferred to
Tennessee. In the
Chattanooga campaign he aided Maj. Gen.
Joseph Hooker in the capture of
Lookout Mountain. Osterhaus then participated in the
Atlanta campaign but a month-long sick leave caused him to miss the crucial
Battle of Atlanta. However, he returned to command and played a significant role in the
Battle of Jonesborough. After the capture of
Atlanta, he received command of the
XV Corps, one of the four corps into which the army was consolidated, in the
Sherman's March to the Sea. In March 1865 Osterhaus was appointed
chief of staff in the
Military Division of West Mississippi under the command of Maj. Gen.
Edward Canby, a commander with little combat experience in high command. Osterhaus served Canby through the battles of
Spanish Fort and
Fort Blakeley. When
Edmund Kirby Smith surrendered the Confederate forces in the
Trans-Mississippi Theater, Osterhaus was sent as Canby's representative and therefore personally signed the documents on behalf of the Union army. He was mustered out of the service on January 15, 1866, and the same year was appointed United States Consul at
Lyon, France, but subsequently made his home in Germany, at
Duisburg. He retired in 1905, and was in 1915 the oldest pensioner on the Army list. Osterhaus died in Duisburg and was buried in Koblenz, Germany. Some thought he was buried at the Koblenz Jewish Cemetery, perhaps as a Carmen Osterhaus, born in the 1850s, had been listed as a Holocaust victim. However, the family vault was instead located at "Der Hauptfriedhof Koblenz" (the main cemetery, or city cemetery, of Koblenz). The crypt no longer exists. Ruined by terrain shifts in 1969, it was then abandoned. In 2012, a marker was erected at the old site, jointly funded by the city of Koblenz and Osterhaus descendants, including biographer Mary Bobbitt Townsend. ==Commemorations==