I was reading this thread and I decided to add my two cents on Mules:
First; you can breed Mules, True, all male mules and most female mules are infertile. But, a mule is the offspring of a male donkey and a female horse; which means all you need is one fertile male donkey and one fertile female horse and you will get a mule.
Second; as for availability of mules, there are dedicated number of mule breeders, continued the tradition of breading as hobby and continued the great lines of mammoth jacks started in the United States by George Washington; with the gift from the King of Spain of two Catalan jacks. Also, Amish farmers, who reject tractors and most other modern technology, commonly use teams of six or eight mules to pull plows, diskers, and other farm equipment.
Third; as for use of mule in the military, currently the US Marine Corps conducts an 11-day Animal Packers Course; which covers the use of mules; at its Mountain Warfare Training Center, located in the Sierra Nevada near Bridgeport, California.
As for other countries using mules check out this picture I found will surfing the Web, I believe Soilder is West German:
