Two famous post-war explorers fought against each other at Shiloh. Who were they? 

Answer:    

John Wesley Powell, who was the first to journey down the Colorado River through the Grand Canyon, and Henry Morton Stanley, who encountered Dr. David Livingstone in Africa, prompting the famous exchange "Dr. Livingstone, I presume?"

Powell ( pictured above ) explored the Grand Canyon with one arm, having lost the other one at Shiloh while commanding an Illinois artillery battery. Stanley was a Confederate infantryman, captured by the Federals at the battle

( Source: NPS.  section updated 01/14/2012