The war in Europe interrupted trade and commerce, threatened American overseas investment and markets, and made clear the costs of neutrality.
Unrestricted Submarine Warfare
The German government announced that it would disregard international law and sink any ships attempting to reach Europe. This put Americans traveling or trading with the Allies in danger.
The Zimmerman Telegram
The British Secret service intercepted and decoded a telegram detailing a proposal by the German Minister of Foreign Affairs, Arthur Zimmermann, that called for an alliance with Mexico against the United States if the latter entered the war.
Public Pressure
American newspapers ran headlines describing atrocities and attacks on civilians, most notoriously of women and children during the German advance into Belgium and France.