Arkaign
Lifer
You're a bit off target there. For quite some time during the early stages of WW2 (before Germany declared war on us), the Kriegsmarine pretty much went out of their way to avoid sinking any US ship, particularly warships. With 30s technology, most of it recycled from WW1 uboats, it wasn't always easy to tell one merchant ship from another.
Considering that the US was the backbone of material support to keep the UK in the war, it's rather surprising they didn't declare open hostilities earlier. It'd be like if we were in a serious war against an enemy here in our hemisphere, and they kept getting war supplies from Japan, most of us would say we would have a right to destroy those vessels.
There are numerous occasions where u-boat commanders would allow the crew to disembark vessels before sinking the ship, or to otherwise offer aid and assistance.
Read this and be astonished :
http://en.wikipedia.org/wiki/Laconia_incident
U-156 remained on the surface at the scene for the next two and a half days. At 11:30am on September 15, she was joined by U-506 commanded by Kptlt. Erich Würdemann and a few hours later by both U-507 under Korvettenkapitän Harro Schacht and the Italian submarine Cappellini. The four boats, with lifeboats in tow and hundreds of survivors standing on their decks, headed for the African coastline and a rendezvous with Vichy French surface warships which had set out from Senegal and Dahomey.[1]
American bombing
The next morning, September 16, at 11:25am, the four submarines, with Red Cross flags draped across their gun decks, were spotted by an American B-24 Liberator bomber from Ascension Island. Hartenstein signalled to the pilot requesting assistance. Lieutenant James D. Harden of the U.S. Army Air Force turned away and notified his base of the situation. The senior officer on duty that day, Captain Robert C. Richardson III, replied with the order "Sink sub."
Harden flew back to the scene of the rescue effort and at 12:32pm attacked with bombs and depth charges. One landed among the lifeboats in tow behind U-156 while others straddled the submarine itself. Hartenstein cast adrift those lifeboats still afloat and ordered the survivors on his deck into the water. The submarines dived and escaped. Hundreds of Laconia survivors perished, but French vessels managed to re-rescue about a thousand later that day. In all, some 1,500 passengers survived.
Under the Hague Conventions, hospital ships are protected from attack, but their identity must be communicated to belligerents (III, 1-3), they must be painted white with a Red Cross emblem (III, 5), and must not be used for other purposes (III, 4). Since a submarine remained a military vessel even if hors de combat, the Red Cross emblem did not confer automatic protection, although in many cases it would have been allowed as a practical matter. The order given by Richardson has been called a possible war crime, but the use of a Red Cross flag by an armed military vessel would be a violation under the Geneva Convention of 1949 (II, 44). There is no provision in either convention for temporary designation of a hospital or rescue ship. Under the informal rules of war at sea, however, ships engaged in rescue operations are held immune from attack.
The Laconia incident had far-reaching consequences. Until then, as indicated in point #1 of the "Laconia Order," it was common for U-boats to assist torpedoed survivors with food, water and directions to the nearest land. Now that it was apparent the Americans would attack rescue missions under the Red Cross flag, Dönitz prohibited rescues; survivors were to be left in the sea.
