When the German submarine, U-30, sank an unarmed British passenger ship on Sept. 3, 1939, a few hours after England had entered the war, the U-boat’s commander said he thought he was attacking a warship.

This was an important distinction because his operational orders forbade him to attack a passenger ship. The Grand Admiral of Germany’s navy at the time, Erich Raeder, believed a German U-boat attack on the British passenger ship Lusitania turned world opinion against Germany in World War I, and he wanted to avoid a similar situation in the coming conflict.

Oberleutnant Fritz-Julius Lemp, U-30’s commander, was well aware of this caution as well as the general restrictions of the London Submarine Protocol, which Germany signed in 1936. The protocol required him to warn any un-armed, un-escorted merchant ship of his intent to attack unless the ship stopped and allowed a boarding party to inspect its cargo. If the cargo contained contraband, the ship could be sunk but only after its crew was evacuated.

To complicate matters, the British Admiralty in the summer of 1939 had begun to convert certain merchant ships into “armed merchant cruisers” by adding naval guns to their decks. These ships were intended to supplement the Royal Navy’s protection of the sea lanes that were critical to the British Isles’ existence.

Oberleutnant Lemp had been warned specifically of the threat posed by armed merchant ships, a concern undoubtedly on his mind as he weighed the decision whether to attack. An armed enemy ship would be a legitimate target that would not require him to give warning.

The protocol was problematic for submarine warfare. Advances in radio transmission, the advent of the aircraft carrier, and the increasing range of aircraft all added to the dangers a submarine faced while on the surface of the water when it was most vulnerable.

The British Admiralty could not be certain Germany would follow procedures set out by the submarine protocol. Days before war was declared, the Admiralty asked all British merchant ships to sail blacked out at night. In the event of war, ships were further advised to sail in a zigzag pattern to make it more difficult for U-boats to target them.

Indeed, the passenger ship Athenia was adhering to the Admiralty’s advice when she was torpedoed without warning by U-30 the evening of Sept. 3.

When Lemp discovered via Athenia’s distress signals that he had torpedoed a passenger ship, he is reported to have remarked, “What a mess,” and wondered aloud why the ship had been sailing blacked-out.

Were Lemp’s remarks self-servingly constructed after the fact, or had he really been shocked to learn of his mistake? And if it was a mistake, did the Admiralty’s directives to merchant shipping contribute to Lemp’s decision to attack?

We will never know the answers to these questions. Oberleutnant Lemp did not survive the war, and nearly all of the members of U-30’s crew who did survive have died in the seven decades since the war ended. While it seems possible the Admiralty’s actions weighed on Lemp’s decision, the British hardly can be faulted for taking such precautions.

In his short time as a U-boat commander (slightly more than ten months), Lemp had acquired a reputation for bravado. But it seems unlikely he would have ignored a standing order prohibiting attacks on passenger ships. We’re left with the most likely conclusion that his eagerness led him to see a legitimate target in his periscope’s cross-hairs on that long-ago September evening. His rash decision forever changed his life and the lives of 1,418 men, women, and children aboard Athenia.