If I'm not mistaken, this ban on umbrellas continues to this day for many military organizations:
Captain Mercer of the British Horse Artillery described the miserable night he and his troop spent on the field of Waterloo before the battle: “My companion (the troop’s second captain) had an umbrella, which by the way afforded some merriment to our people on the march, this we planted against the sloping bank of the hedge, and seating ourselves under it, he on the one side of the stick, me on the other side, we lighted cigars and became-comfortable”.
The Duke, who was indifferent to the way his officers chose to dress, drew the line at umbrellas. “At Bayonne, in December 1814,” writes Captain Gronow of the First Foot Guards, “His Grace, on looking round, saw, to his surprise, a great many umbrellas, with which the officers protected themselves from the rain that was then falling. Arthur Hill came galloping up to us saying, Lord Wellington does not approve of the use of umbrellas during the enemy’s firing, and will not allow the “gentlemen’s sons” to make themselves ridiculous in the eyes of the army.”
Colonel Tynling, a few days afterwards, received a wigging from Lord Wellington for suffering his officers to carry umbrellas in the face of the enemy; His Lordship observing, “The Guards may in uniform, when on duty at St. James’, carry umbrellas if they please, but in the field it is not only ridiculous but unmilitary.”
Standing orders for the army in the Peninsula and in the Waterloo campaign stated categorically “Umbrellas will not be opened in the presence of the enemy.”
However the surgeon of Captain Mercer’s troop of Horse Artillery was seen to be sheltering under the forbidden item during the early part of the Battle of Waterloo.Excellent stuff. The umbrella image from above is from the collection of the Museum of Fine Arts in Boston, and it's a lady's parasol. This one was made, apparently, in England in the late 18th or early 19th century. It does correspond to the time period, however.