briefly explain why the German submarine policy was the most significant factor contributing to American involvement in World War I

In Atlantic waters, a German U-boat launched a torpedo and sank the British liner Lusitania on May 7, 1915. 1.198 people died, including 128 Americans. The Germans followed a policy of indiscriminated attack and sinking of vessels and ships that could be transporting goods for Britain and other allies, no matter the neutrality of a third nation. The US was formally neutral still in 1915. This event outraged the American public and is considered one of the factors that pushed the USA to join the war in 1917.