What was “the mad story of Detering”? Explain.

I'm not sure what you mean by "mad". Detering does sort of lose it and he goes AWAL. He also can't stand the sounds of horses dying.

Detering misses his wife back home on his farm in Oldenburg. A kind-hearted nature lover, Detering's biggest emotional outburst comes over the pains dying horses in the war. As the horses scream in agony, Detering yells, "God! For God's sake! Shoot them! […] Like to know what harm they've done" (4.60-71). He is a clever rat-catcher, having been the one to make the long wire dangler with a hunk of bread on the end of it. One day the men come across a flowering cherry tree. Detering is moved beyond belief. In the wee hours of the night, Paul wakes to find him packing. Two days later, Detering is nowhere to be found; he has run away. The men later discover that Detering has been found and court-martialed. He is never seen again.