At the outbreak of the war in 1914, the United States, under President Woodrow Wilson, declared a policy of strict neutrality.

However, American public opinion gradually shifted in favor of the Allies due to cultural ties to Britain and reports of German atrocities.

The U.S. had strong economic ties to the Allies, providing them with loans and supplies.

Germany's policy of unrestricted submarine warfare and the sinking of American ships turned public opinion sharply against Germany.

The sinking of the British liner Lusitania in 1915, with 128 Americans on board, was a major catalyst.

The final trigger for U.S. entry was the interception of the Zimmermann Telegram in 1917.

The Zimmermann Telegram was a secret message from Germany to Mexico proposing a military alliance against the United States if the U.S. entered the war.

The publication of the telegram caused outrage across America.

President Wilson asked Congress to declare war on Germany in April 1917, stating that 'the world must be made safe for democracy.'

The entry of the United States, with its vast industrial resources and fresh troops, was a decisive factor in the eventual Allied victory.