This Germania design for a U-cruiser was the largest type of submarine to enter service with the German Fleet during World War I. The supplemental diesel generator provided dash speed on the surface. Twenty-seven larger versions of this type were ordered, but none had been completed before the end of the war; all were scrapped in 1919–1920. The very long range of these boats and their fine performance, particularly on the surface, was very influential on postwar submarine design, especially in Japan and the United States. All three boats survived the war. The U- 129 went to France and served as the Halbronn until stricken in 1935; the U-140 went to the United States and was sunk as a target in 1921; and the U-141 went to Britain and was scrapped in 1923.
All-out submarine warfare by Germany famously and nearly brought the United States into World War I in 1915, and in fact forced its belligerency in 1917. It also very nearly crippled Great Britain. Those related facts set up an essential tension in German naval thinking between the world wars that was not fully resolved in favor of all-out combat until U.S. entry into the war. By that time, the Kriegsmarine had lost the Battle of the Atlantic (1939–1945) in terms of its own metric of tonnage sunk per day. On the other side, no amendments were made to established rules of cruiser warfare between the world wars, partly because the British thought they had ensured an end to the German U-boat arm under terms of the Treaty of Versailles (1919) and the surrender of over 200 U-boats in 1918. London later vitiated its naval interests by agreeing to the Anglo-German Naval Agreement in 1935, an error not saved by the merely symbolic bandage of the London Submarine Agreement of 1936, which ostensibly confined U-boats to operating under cruiser or prize rules. Fortunately for the British, Adolf Hitler concentrated on building a fleet of battleships, battlecruisers, and heavy cruisers under the Z-Plan , when the Kriegsmarine should have been building a vast fleet of submarines as Admiral Karl Dönitz argued. Dönitz had headed a secret group within the High Command that spent the 1920s in U-boat research and planning. He was convinced that the U-boat could be a war-winning weapon for Germany if he was allowed to fight with a fleet of at least 300 operational boats. Dönitz was committed to total war at sea, but Hitler held him and his eager U-boat captains back in the first days and months of the war. Why?
The Kriegsmarine was far less restricted in its U-boat operations right from the start of the war in 1939 than the Kaiserliche Marine had been in 1914. Hitler was at first mindful of naval lessons of the Great War, although he hardly understood sea power at all. Rather than authorize total war at sea from the first moment he declared French shipping off limits to attack. He ordered that no passenger liners should be sunk even if they were blacked out and found zigzagging or taking other defensive measures, or even when they were traveling in convoy. Within just hours, however, the unarmed liner “Athenia ” was sunk in error by an eager U-boats captain. Hitler immediately reiterated the ban on sinking liners out of real concern for provoking American and other neutral opinion with another “Lusitania” incident. He lifted it partly on September 23, then reimposed it, in an erratic sequence of decisions that spoke to persistent tension in Germany’s approach to war at sea as well as to Hitler’s fundamental and persistent failure to understand sea power. While some U-boat captains champed at the bit, straining to ruthlessly “sink on sight” any ship they spotted, others observed the rules: they waited after an attack, helped wounded sailors, and provisioned lifeboats as best they could. This was still possible because British and Commonwealth navies did not yet have enough escorts or weapons to make humane behavior by U-boat captains imprudent or always fatal. Once Allied anti-submarine warfare techniques improved and more and better escorts went to sea, chivalry ceased. A related factor was that American public opinion on the question of unrestricted submarine warfare was different than during World War I. From 1939 to 1941 isolationists militated against protesting “sink on sight” practices, lest Americans again became agitated to belligerence by assertions of violated “neutral rights” and stories of German atrocities at sea. U.S. citizens were instead told bluntly that they steamed into War Zones or took passage on Allied ships solely at their own risk.
After the war, Dönitz was convicted by the Nuremberg Tribunal as a “major war criminal” on the charge that he had ordered U-boat captains to ignore rules of cruiser warfare, to not stop or rescue crews from sunken ships. That was factually true. However, he was only censured for that order by the Tribunal. His ten year sentence was imposed for helping to plan a war of aggression, not for practicing unrestricted submarine warfare. It was the lightest such sentence received by any major defendant. Why such lenient treatment? Because even hypocrisy has its limits: many Allied navy men knew that they might have taken Dönitz’s place in the dock had their side lost the war. Immediately upon entry into the Pacific War, on the afternoon of December 7, 1941, President Franklin Roosevelt ordered the U.S. Navy to “sink on sight” all Japanese shipping in the Pacific theater. The USN implemented that core policy of unrestricted submarine warfare to devastating effect from 1942 to 1945. The British had employed sink on sight tactics in the Mediterranean and elsewhere.