The primary factor in the U.S. decision to enter World War I in 1917 was the unrestricted submarine warfare policy adopted by Germany. This aggressive strategy led to the sinking of civilian and neutral ships, including the British ocean liner RMS Lusitania in 1915, which killed 128 Americans. Additionally, the interception of the Zimmermann Telegram, wherein Germany proposed a military alliance with Mexico against the U.S., heightened tensions. Economic interests also played a role, as American banks and businesses had significant investments in the Allies. These factors combined to push public opinion and government leaders towards supporting entry into the war.