America joined the war on April 6th 1917 with the declaration of war against Germany. For three years, America remained neutral. Why? In the beginning part of the war, many Americans were pro-German. The president Woodrow Wilson was very much against the idea of involving USA in a foreign war. He did not want it to disrupt his presidency. Many Americans were opposed to the idea of joining any conflict(they were isolationists). However, they supplied materials to both sides of the war. This trade significantly helped the American economy.$^0$
There were five main reasons that the United States entered the war.$^{10}$
The RMS Lusitania was a passenger liner that left New York heading to Liverpool. However it was sunk by a German U-boat. 128 American passengers were killed as a result of this attack. This caused wide spread outrage in America.$^{10}$
Stories of the atrocities committed by the Germans against the Belgians during their invasion of Belgium began to spread. Both the true and exaggerated stories were used as part of propaganda. This propaganda painted Germany as a barbaric and ruthless nation that was hell bent on destruction. This propaganda reached the US, sparking anti-German sentiment, and reduced the pro-German sentiment.$^{10}$
The United States had a vested interest in the Allied Powers(Triple entente) winning the war. This is because many American businesses and banks made large loans to the Allied Powers. Should the Central Powers win the war, they were unlikely to get their money back.
After Lusitania was sunk in 1915, Germany removed the unrestricted submarine warfare policy. However it was reintroduced in 1917. Germany gambled that they would defeat the British before the United States mobilized. Many US cargo vessels were sunk without warning. This led to the US severing diplomatic ties with Germany.$^{10}$
Unrestricted submarine warfare means that the submarines will attack merchant ships without warning.$^{11}$
The fifth and final reason was the Zimmerman telegram. It was sent from the German Foreign Secretary, Arthur Zimmermann to the German diplomatic representative in Mexico. It proposed a secret alliance between Germany and Mexico, should the United States enter the war, that promised that Mexico will be able annex the United States territories of New Mexico, Arizona, and Texas. It was, however, intercepted by the British and then decrypted by Room 40. This was sent to the United States and was released to the public.$^{10}$