POWELL, Wyo. — Mother Nature has bested billions of dollars worth of 21st-century defense technology, as U.S. soldiers in Afghanistan are now counting on donkeys to help move troops and supplies over steep mountain trails. But with the last major use of pack animals having ended more than 60 years ago, today’s Army is short on experts in the art of muleskinning, or wrangling mules and donkeys. Which is why 31 soldiers from the Army’s 10th Mountain Division spent much of the week in a barn at the foothills east of Powell, learning the finer points of donkey management. The...