The United States entered World War I on April 6, 1917, primarily due to Germany's resumption of unrestricted submarine warfare, which threatened American lives and commerce, and the interception of the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the US. President Woodrow Wilson, who initially aimed to keep the US neutral, asked Congress to declare war to "make the world safe for democracy," framing the conflict as a moral responsibility and a necessity to protect US interests. Additionally, growing American public opinion had shifted against Germany after incidents like the sinking of the ocean liner RMS Lusitania, which killed American civilians. Key factors include:
- Germany resumed unrestricted submarine warfare in 1917, sinking American merchant ships.
- The Zimmermann Telegram exposed Germany's proposal to Mexico to join the war against the US.
- Wilson's idealistic vision of a "war to end all wars" to protect democracy.
- The economic ties to the Allies, with American banks heavily lending to Britain and France.
- Shifting public opinion increasingly viewing Germany as the aggressor after earlier German atrocities in Europe.
The US entry marked a turning point by bringing fresh troops, supplies, and money that helped tilt the balance in favor of the Allies.
