Adolf K.G.E. von Spiegel commanded a German U-boat during the First World War.
Here is a portion of his memoirs where he describes the attack on a cargo vessel in
April 1916.
"The steamer appeared to be close to us and looked colossal. I saw the captain walking on his bridge,
a small whistle in his mouth. I saw the crew cleaning the deck forward, and I saw, with surprise and
a slight shudder, long rows of wooden partitions right along all decks, from which gleamed the
shining black and brown backs of horses."

'Oh heavens, horses! What a pity, those lovely beasts!'

'But it cannot be helped,' I went on thinking. 'War is war, and every horse the fewer on the Western
front is a reduction of England's fighting power.' I must acknowledge, however, that the thought of
what must come was a most unpleasant one, and I will describe what happened as briefly as
possible."

'Stand by for firing a torpedo!' I called down to the control room.'

'FIRE!'

" A slight tremor went through the boat - the torpedo had gone."

"The death-bringing shot was a true one, and the torpedo ran towards the doomed ship at high speed.
I could follow its course exactly by the light streak of bubbles which was left in its wake."
"I saw that the bubble-track of the torpedo had been discovered on the bridge of the steamer, as
frightened arms pointed towards the water and the captain put his hands in front of his eyes and
waited resignedly. Then a frightful explosion followed, and we were all thrown against one another
by the concussion, and then, like Vulcan, huge and majestic, a column of water two hundred metres
high and fifty metres broad, terrible in its beauty and power, shot up to the heavens."

'Hit abaft the second funnel,' I shouted down to the control room."

"All her decks were visible to me. From all the hatchways a storming, despairing mass of men were
fighting their way on deck, grimy stokers, officers, soldiers, groom, cooks. They all rushed, ran,
screamed for boats, tore and thrust one another from the ladders leading down to them, fought for
the lifebelts and jostled one another on the sloping deck. All amongst them, rearing, slipping horses
are wedged. The starboard boats could not be lowered on account of the list; everyone therefore ran
across to the port boats, which in the hurry and panic, had been lowered with great stupidity either
half full or overcrowded. The men left behind were wringing their hands in despair and running to
and fro along the decks; finally they threw themselves into the water so as to swim to the boats."

"Then - a second explosion, followed by the escape of white hissing steam from all hatchways and
scuttles. The white steam drove the horses mad. I saw a beautiful long-tailed dapple-grey horse take
a mighty leap over the berthing rails and land into a fully laden boat. At that point I could not bear the
sight any longer, and I lowered the periscope and dived deep."