Peter Joseph Osterhaus was a German-American Union Army general in the American Civil War and later served as a diplomat.

The Battle of Pea Ridge, also known as the Battle of Elkhorn Tavern, took place during the American Civil War near Leetown, northeast of Fayetteville, Arkansas. Federal forces, led by Brig. Gen. Samuel R. Curtis, moved south from central Missouri, driving Confederate forces into northwestern Arkansas. Maj. Gen. Earl Van Dorn had launched a Confederate counteroffensive, hoping to recapture northern Arkansas and Missouri. Confederate forces met at Bentonville and became the most substantial Rebel force, by way of guns and men, to assemble in the Trans-Mississippi. Against the odds, Curtis held off the Confederate attack on the first day and drove Van Dorn's force off the battlefield on the second. By defeating the Confederates, the Union forces established Federal control of most of Missouri and northern Arkansas.