The United States’ involvement in World War is very complex, because of both the overwhelming amount of propaganda surrounding it, and the bizarre character of President Wilson.
- Britain and America had a close trade relationship, but tension arose between the U.S. and Germany when German mines damaged some American ships on their way to Britain.
- Germany declared that all ships, neutral or otherwise, had the potential to be attacked when entering the warzone around Britain. After Germany took responsibility for sinking the William P. Frye, a private American ship that was carrying grain to England, President Wilson was furious. The German government released an apology and assured that the attack was an accident.
- In May 1915, some New York newspapers printed a warning written by the German embassy in Washington, saying that Americans who travel on British or Allied ships through war zones do so at their own risk. The warning was printed on the same page as an advertisement for the upcoming voyage from New York to Liverpool of the British ocean liner, the Lusitania. On May 7, the Lusitania was attacked without forewarning by a German submarine off the coast of Ireland. Out of nearly 2,000 passengers, 1,201 were killed, 128 of which were Americans.
- It was later found that the Lusitania was carrying 173 tons of war armaments for Britain, which the Germans cited as justification for their assault. The U.S. sent three letters to Berlin disputing the action, and Germany apologized and agreed to end their open submarine warfare. However, that November, an Italian liner was sunk by a U-boat with absolutely no warning. 272 people were killed, 27 of which were Americans. The United States opinion of Germany was growing lower and lower.
Later in March, Germany attacked four more of America’s merchant ships. On April 2, President Wilson spoke before Congress and called for war on Germany. On April 4, Senate voted to declare war. Two days later, the House of Representatives backed up this vote, and America officially joined World War I.
Reference:
http://www.telegraph.co.uk/history/world-war-one/inside-first-world-war/part-nine/10801898/why-america-joined-first-world-war.html