The sinking of the Lusitania is remembered as one of the atrocities of World War I that killed a large number of American civilians. People often think that the sinking propelled the US into the war. However, this was not true. The Lusitania was sunk in 1915 and the US did not enter the war until almost two years after the sinking. The sinking of the Lusitania and the American reaction to that sinking caused the Germans to temporarily halt their policy of unlimited submarine warfare.
The Germans had been using submarines to sink shipping that was headed to Britain. By law, they were supposed to warn the ships and give the people on board time to escape. However, when they tried doing this, the British did things like putting weapons on freighters and destroying submarines that surfaced. Therefore, the Germans stopped playing by the rules.
The US was very unhappy with unlimited submarine warfare as it was killing American civilians. The German government did not want the US in the war so, about a year after the sinking of the Lusitania, they issued the “Sussex Pledge” saying they would stop this kind of warfare. This did not last very long, however, as the Germans decided they had to start unlimited use of submarines again if they were to have any chance of winning the war.
So, the sinking of the Lusitania led to American protests to Germany, which led to the temporary suspension of unlimited submarine warfare.