Final answer:
The United States' stance of neutrality was challenged in 1915 due to Germany's policy of unrestricted submarine warfare and the Zimmerman Telegram, leading to U.S. entry into World War I in April 1917.
Step-by-step explanation:
In the historical context of World War I, the United States initially adopted a position of neutrality, avoiding involvement in the escalating European conflict. This stance was seriously tested in 1915 when Germany began its policy of unrestricted submarine warfare, targeting and sinking ships without warning, including those of neutral countries like the United States. The situation escalated when the Zimmerman Telegram was intercepted, in which Germany proposed a military alliance with Mexico against the U.S., and U.S. ships began to be sunk by German U-boats.
These aggressive acts challenged the U.S. commitment to neutrality and isolationism, leading to increasing public outrage and a shift in sentiment towards supporting the Allied powers. By early 1917, strategic concerns regarding the possible German victory and Bolshevik seizure of power in Russia became pressing. Germany's miscalculation that the U.S. wouldn't be able to mobilize quickly enough for war proved incorrect, and in April 1917, following the sinking of more U.S. ships and the public release of the Zimmerman Telegram, President Woodrow Wilson asked Congress to declare war, thus ending American neutrality.