World War I saw submarines used effectively on a large scale for the first time. Germany realized that it could not challenge the British Navy with its surface fleet and so attempted to cut the British isles off from food and raw materials from its colonies using its submarine fleet. This attempt had unforeseen diplomatic consequences, however. On March 24, 1916, the French passenger liner Sussex was sunk without warning. The sinking of passenger vessels angered the United States, and so Germany issued the Sussex Pledge on May 4th of 1916, promising not to target passenger ships and not to target merchant ships that did not carry weapons.
German submarines had begun operating in the first year of the war. For most of 1914 the main targets were warships as the Germans thought that the U-Boat was a flawed commerce raider and did not want to anger neutral powers. German and Austrian submarines were able to sink many allied cruisers and even a few battleships. However, the Allies developed a pattern of erratic movement that if followed by a ship would prevent its targeting by a U-Boat. Attacks on merchant ships began in October, but only 19 were sunk until February of 1915. In 1915, Germany began more aggressively targeting commercial ships in reaction to the harsh British blockade imposed on Germany. Most famously, RMS Lusitania was sunk. Although it was later discovered that the ship had been carrying weapons, the US public was outraged at the death of American citizens. German and Austrian submarines sank military ships in the Atlantic, the North Sea, and the Mediterranean and greatly increased the number of commercial vessels sunk. The practice of sinking civilian ships continued, but the US was angered when a passenger liner, the French Sussex, was sunk. Germany wanted to prevent the US from entering the War, and so issued the Sussex Pledge. The Pledge detailed that passenger ships would not be attacked, that merchant ships would not be sunk unless they carrier weapons, and that the crew of merchant ships would be allowed to escape the ship before sinking.
While these measures pacified the US, they weakened the German war effort. In 1917, Germany decided to resume unrestricted submarine warfare and was able to sink 25% of all British shipping by March. However, the US declared war in response to the resumption and along with the British were able to bring shipping losses down to acceptable levels by the end of 1917. In 1918, Germany would suffer ever heavier submarine casualties and would be unable to inflict crippling damage on allied shipping thanks to countermeasures like improved convoy systems, the use of airplanes to spot submarines, and depth charges. In the end, German efforts to starve Britain failed to be as effective as the British blockade on Germany. Germany’s attempt to win a protracted war would fail because it could not feed its own population or supply its factories with natural resources. Further, Britain’s superior navy was able to blockade Germany and protect her supply lines at the same time. The cruel truth of German warfare, that Germany will loose if the war drags on from more than two years due to lack of resources, played out in World War I and again in World War II, and is one of the many reasons why Germany has not started a war since.