What were submarines used for in world war 1?

Answer 1

Same as WW2 but there were not as many of them.
As with any other warship, submarines were used to attack the ships and other military targets of the enemy. However, submarines had the advantage of stealth and could often avoid detection until they attacked.

In World War I the German Navy, outgunned by the British Navy, relied on their U-boats to sink merchant vessels (armed or unarmed) carrying military supplies to and from Great Britain and France. This "unrestricted submarine warfare" became one reason that the US entered the war, as American crews and passengers were being killed by these attacks.

This same tactic was employed by the US in World War II, sinking Japanese ships and cargo transports to disrupt enemy supply lines.
The biggest factor in submarine warfare during WWI was Germany's introduction of Unrestricted Submarine Warfare, a tactic in which no warning is given to the target vessel prior to its being attacked. Prior to this, a method known as Prize, or Cruiser, rules were used; however, those rules stated that the attacking vessel must warn the target vessel ahead of time and allow the crew to abandon the vessel before its destruction. With the advent of the modern Diesel Electric submarine at the time, a submarine Captain using Prize rules would be a sitting duck, since the target vessel could easily signal a friendly warship to come to its aid.

The attack on the RMS Lusitania illustrated the effect of USRW; though the Germans suspended the tactic after the attack due to global criticism, they resumed again in 1917 to try and break the British blockade of Germany that was having a major effect on the German war effort. They failed; in retrospect, had they continued with their initial tactic of URSW, the war may have turned out differently, since real ASW tactics weren't devised until WWII.

The Allies viewed the submarine threat so great that the Treaty of Versailles, which ended WWI, stipulated that all German submarines were to be turned over to the Allies. This was both for military and technological reasons; the Allies wanted to get their hands on German submarine technology, and they wanted to prevent the Germans from using them again. The same thing happened at the end of WWII; many designs still in use today on modern DE and Nuclear Powered submarines are a direct result of WWII German submarine technology from U-Boats captured or surrendered at the end of the war.

The doctrine of USRW was used to devastating effect by the United States in the Pacific against Japan after the attack at Pearl Harbor. Submarines were the only front line defense units after Pearl Harbor, and kept the Japanese busy until the fleet could be restored and returned to the fight. Though many view it as a tactic that shouldn't be used, there is no question that the mere threat of USRW has a great impact on how Navies of all nations prepare their battle strategies. Its introduction in WWII forever changed the way submarines are used in warfare.
"Guerre de Course", commerce raiding.

Answer 2

They were responsible for the Lusitania disaster, which was no accident but a deliberate torpedo attack- casualties in four figures, and drawing the US indirectly into the war two years later, as Americans ( civilians mainly) were passengers lost in the Ship sinking, which was understood to be a sub attack, as stated and not any form of accidental impact- like the Titanic.

Answer 3

Submarines were used in WW1 to attack convoys carrying supplies, sinking the enemy navy, and reconnaisance. These tactics were used again in WW2 by both the German and American navies; the Germans were almost successful in closing the Atlantic to the US supplying Britain, whereas the American submarine fleet was an integral part of sinking Japan's navy, her entire cargo fleet, and subsequent blockade of Japan prior to the Japanese surrender.