Well, what started the tensions was the sinking of the Lusitania by a German U-Boat in the Celtic Sea, in response to this U.S. President Woodrow Wilson sent letters to the german government demanding they stop attacking innocent merchant ships.
Kaiser Willhelm, the ruler of Germany at the time, issued an apology to the United States out of fear of them joining the war. Willhelm altered Germany's aggressive naval policy but under the pressure of his military leaders soon reverted back to their aggressive stance.
On the 3rd of Febuary 1917 the American liner Housatonic was attacked by German U-boats and sunk. As a result, America formally declared war on Germany April 6, 1917.
