Peter Joseph Osterhaus was one of the North's most competent commanders, yet one of the least known, for several reasons. First, he fought in the Western Theater, less well covered than the war in the East by the press at the time and by later historians. Second, he was an outsider: although a naturalized American citizen, he was the farthest thing from a West Pointer, never losing his heavy German accent and never quite "getting" American humor or sarcasm. Third, most of his commanders were not highly regarded for one reason or another: Sam Curtis, Franz Sigel, John Frémont, John McClernand, Joseph Hooker, and E. R. S. Canby. Fourth, he wrote very little about his experiences: a twenty-five page war memoir and one year of his diary are all that now exist. Yet the fact remains that he was a major contributor to Union success in the West.
It is sometimes written that he was an experienced Prussian officer. Well, sort of, but most of his expertise was gained in the field in the United States rather than in officer's training. His Prussian training consisted of one year of drill plus six months in the reserves as a second lieutenant. (The Prussian overlords required all males in the Rhine Province, where he was born, to serve a year.) His real military education began in the trenches of Mannheim in the Revolution of 1848-9, as he tried his hardest to whip fellow rebels into some sort of an army to withstand these same Prussians, massing on the border of Baden.
Having fled to the United States in 1849 after the collapse of the revolt, the last thing on Osterhaus's mind was more fighting. He established himself as a businessman in Belleville, Illinois, joined the Republican Party, and for the next ten years made himself into a reasonably successful American citizen, including a stint as postmaster of nearby Lebanon. But that peaceful existence was to change: Moving to St. Louis for financial reasons in late 1860, he was soon embroiled in the lead-up to the Civil War, doing his part by secretly training medical students to be militiamen in order to protect the vital Federal arsenal there.
Shortly after Ft. Sumter, he enlisted as a 37 year-old private in the 2nd Missouri Volunteer Infantry, and was rapidly elected captain and then major by the end of two weeks. His skill in training recruits was patent; being bilingual made him even more valuable to his commanding officers, since most of his fellow recruits in his 3-month regiment were German-speaking. Even so, from this point, his rapid advancement was striking: most of his fellow German-American volunteers never made it past regimental command while he was a brigadier general after only a year.
After Maj. Osterhaus's strong performance commanding a rifle battalion at Wilson's Creek, Frémont appointed him a colonel and ordered him to recruit a 3-year regiment: the 12th Missouri Volunteers. Osterhaus trained this unit for about a month before taking them on campaign. But once on the road, his time with them was short: within a few days he was bumped up to brigade command and by the next big battle, Pea Ridge, he was commanding a large, independent detachment in the crucial part of that battle at Leetown.
From that point until nearly the end of the war, Osterhaus commanded at the division level, honing his skills along the way. (He was particularly noted for his skill in artillery placement and its innovative use with skirmishers.) He made significant contributions in several of the major campaigns of the war: Vicksburg, Chattanooga and Atlanta. See Appointments and Engagements for some of the details. As a new major general, in the fall of 1864 he assumed command of the XV Corps of the Army of the Tennessee under O. O. Howard and Sherman for the chase after John Bell Hood and the March to the Sea. Once Savannah was taken, he was reassigned as chief of staff to E. R. S. Canby, a commander with no battle experience at higher than regimental level, and contributed his own hard-won expertise to the successful capture of the forts at Mobile Bay, the last action of the war. He was proud to sign the final surrender of the Trans-Mississippi Confederates, representing Canby while Simon Bolivar Buckner signed for E. Kirby Smith.
Immediately after the war, Osterhaus spent six months in the bruising assignment of military governor of Mississippi during Presidential Reconstruction. Within six months of his discharge he was on his way back to Europe as U. S. consul to Lyon, France, a post he held for the next eleven years (which included the Franco-Prussian War). Then, after twenty-five years in private life in Mannheim, Germany, he won his last diplomatic position: U. S. vice-consul to Mannheim. He finally retired in 1900 and lived seventeen more years, dying in Germany just before his son, Rear Admiral Hugo Osterhaus, commander in chief of the Atlantic Fleet in 1912, came out of retirement to assist in the war effort against his father's native land. Shortly before Osterhaus died he was appointed Major General, U. S. Army, becoming the last survivor of the Civil War at this rank.