Many attribute it to the sinking of an American ship, the Lusitania. I think it was a little more complicated because by then we were basically supplying the British, but when the German subs shot that down, that was the last straw.

http://www.answerbag.com/q_view/130017/410453

another reason, germany or japan gave a telegram to mexico that if they would attack the US. they will help mexico get texas, nevada, cali, and other states US took from Mexio

The United States originally pursued a policy of isolationism, avoiding conflict while trying to broker a peace. This resulted in increased tensions with Berlin and London. When a German U-boat sank the British liner Lusitania in 1915, with 128 Americans aboard, U.S. President Woodrow Wilson vowed, "America was too proud to fight" and demanded an end to attacks on passenger ships. Germany complied. Wilson unsuccessfully tried to mediate a settlement. He repeatedly warned the U.S. would not tolerate unrestricted submarine warfare, in violation of international law and U.S. ideas of human rights. Wilson was under pressure from former president Theodore Roosevelt, who denounced German acts as "piracy". Wilson's desire to have a seat at negotiations at war's end to advance the League of Nations also played a significant role. Wilson's Secretary of State, William Jennings Bryan, resigned in protest of the President's decidedly warmongering diplomacy. Other factors contributing to the U.S. entry into the war include the suspected German sabotage of both Black Tom in Jersey City, New Jersey, and the Kingsland Explosion in what is now Lyndhurst, New Jersey. -WIK

In addition to the above: (1) Germany reversed its policy and resumed attacking neutral and merchant ships, even un-armed ships(Jan.1917). Though they knew this ran the risk of angering America, they expected to defeat Allies before America can mobilize and join war (2) The Zimmerman Telegram (1917) - German Foreign Minister Arthur Zimmerman sent a telegram to German ambassador in Mexico. Tell Mexico to join Germany against U.S. and get back Texas, New Mexico and Arizona after war. The British intercepted the telegram, the newspapers published it and American sentiment turned sharply to support entering the war. (3) More American ships sunk (March 1917) (4) The Russian Revolution (March 1917). Kerensky government was a democracy. Wilson now positioned U.S. entry as a war of democracies (which now included Russia) versus autocracies: "The world must be made safe for democracy"

http://www.answerbag.com/q_view/110584

The year was nineteen dickety-two, we had to use the word dickety,because the Kaiser stole our word for twenty. I chased him to get it back, but gave up after dickety-six miles.

Because it was ending!

http://www.geocities.com/Pentagon/Bunker/3017/index.html

To pay back France for helping us out in the Revolution and War of 1812, and to ingratiate them and turn the tables on England and France in the future break-up of the world after the war, strengthening the U.S. position as a major power in world politics.