However, several events and factors changed this initial neutrality and brought about America's entry into the war in April 1917:
1. Lusitania Sinking: In May 1915, a German submarine sank the British passenger liner RMS Lusitania, resulting in the deaths of 128 Americans. This incident caused a public outcry in the US and increased anti-German sentiments.
2. Unrestricted Submarine Warfare: In February 1917, Germany resumed unrestricted submarine warfare in the Atlantic, despite American warnings. German U-boats began sinking merchant ships without warning, causing immense human and material losses.
3. Zimmermann Telegram: In January 1917, the British Secret Service intercepted and decoded a telegram (the Zimmermann Telegram) from German Foreign Secretary Arthur Zimmermann to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico against the US in the event of an American declaration of war on Germany. The telegram's exposure further fueled American anger and resentment towards Germany.
4. Economic Interests: The US had significant economic ties with the Allied Powers (France, Britain, and Russia) and extensive investments in Allied war efforts. The disruption caused by unrestricted submarine warfare threatened American economic interests and prompted a desire to protect these investments.
5. Humanitarian and Moral Considerations: The atrocities committed by Germany during the war, including the widespread destruction, civilian casualties, and use of chemical weapons, shocked and horrified the American public. Many in the US felt compelled to intervene for humanitarian reasons and to uphold democratic ideals and human rights.
By April 1917, these cumulative factors had changed American public opinion and pushed President Wilson to abandon neutrality. The US declared war on Germany on April 6, 1917, formally joining the Allies in World War I.