U-123 sank the Spanish-registered motor ship Castillo Montealegre on 8 April 1943 west of Conakry, French Guinea. As per maritime rules, the neutral ship had the Spanish flag painted in both sides. Commander Horst von Schroeter ordered the shooting of 3 torpedoes and she sunk in less than a minute. The submarine surfaced, the commander asked from the conning tower "What ship?" to the survivors. Although being confirmed he had just sunk a neutral ship, he left without giving any assistance to the 40 men adrift (five went down with the ship).
A few days later the Hill-class naval trawler HMS Inkpen rescued 29 survivors from a boat. 11 on a separated raft died. The affair was hushed-up by the government of Franco; indeed, the survivors were ordered to shut-up. The career of Commander Horst von Schroeter was unaffected by this affair and after the war he even became a NATO commander.
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