Before US Involvement When the war first broke out in Europe everyone assumed it would be over very quickly. The soldiers waved goodbye to their families and said they would be home by Christmas in the summer of 1914. The war very quickly reached a stalemate and was continuing on many years later. The U.S. insisted on staying neutral.
The first incident. Germany was practicing a policy of unrestricted submarine warfare this was when their submarines would sink any ship around British waters without warning. On May 7, 1915 a German u-boat sunk a British passenger ship called the Luisitania. 1,198 people died, 128 of which were American citizens.
US Response The American public was outraged by the Lusitania incident, even though they had been carrying weapons and ammunition. President Woodrow Wilson made sure Germany knew how angry the American people were over this. After two more attacks Germany agreed to stop attacking neutral ships.
Germans Gamble In 1917 Germans began unrestricted submarine warfare again. They thought if they could just blockade Britain long enough to starve them out they could beat them before the US became involved. They sank three American ships.
The Zimmerman Note In February 1917 German actions finally pushed America into the war. The British intercepted a telegram from Germany’s foreign secretary, Arthur Zimmerman, that was intended to go to Mexico. The message said that if the Mexicans joined the war on the side of the Central Powers Germany would then help them reclaim land they had lost to the US. The Zimmerman note was made public, and America (who mostly sided with the Allies already) became enraged and demanded we go to war with Germany. On April 2, 1917 President Wilson asked Congress to declare war on Germany, and we entered on the side of the Allies.