What event led to the U.S. entering World War I?

The decision for the United States to enter World War I was significantly influenced by the practice of unrestricted submarine warfare employed by Germany. This involved German U-boats sinking ships without warning, including civilian passenger liners. One of the most notable incidents was the sinking of the RMS Lusitania in 1915, which resulted in the deaths of 128 Americans. These attacks on neutral ships caused outrage in the U.S. and shifted public opinion in favor of joining the war. Americans were increasingly concerned about the safety of their vessels and the lives of their citizens. Furthermore, Germany's continued policies of unrestricted submarine warfare contributed to diplomatic tensions, ultimately prompting the U.S. government to declare war on Germany in April 1917. This context illustrates how the direct threat to American lives and commerce acted as a catalyst for U.S. involvement in the conflict.