I heard this story years ago, but this is the first time I've seen the actual evidence. Mules are amazingly smart. The reason they are stubborn, is because they have a tendency to think for themselves, as this one clearly did in defending it's "herd."
Gulf War: A Florida reservist says an emotional goodbye to his family before being deployed. The photo is part of "The American Soldier — A Photographic Tribute to Soldiers and Marines" exhibit at the Gerald R. Ford Museum