Why Did the United States Enter the First World War?

Economic Reasons
There were many American businessmen who supported American intervention in the war to further their economic interests. They supported the war efforts of the French and British by providing billions of dollars in loans and bonds. Therefore, many powerful businessmen, such as J.P. Morgan, supported the war and campaigned for the U.S. to enter it and fight alongside the Allied forces.

German Attacks on American Ships
U.S. President Woodrow Wilson initially sought to remain neutral when World War I began in 1914. However, when Germany created a naval blockade of England, friction with Germany intensified and hindered England's trade with the U.S. By 1915, Germany announced that it would attack any ships trying to run the blockade. In early 1915, Germany sank the William P. Frye, which was a private U.S. ship. Germany later apologized and called the sinking a mistake. Hoping to end the war sooner, Germany reneged on its pledge by February 1917 and resumed attacks on shipping vessels. Shortly afterward, an American ship, the Housatonic, was torpedoed and sank by a German U-boat. By this time, the U.S. Congress passed a bill for $250 million in armaments to prepare the country for war. By the next month, Germany had sunk four more American merchant ships.

Sinking of the RMS Lusitania
The RMS Lusitania was a British passenger ship that was sunk by torpedoes fired from a German U-boat when it was off the coast of Ireland on May 7, 1915. There was military equipment being carried aboard the ship, but the Germans sank the Lusitania without warning, resulting in the deaths of more than 1,200 passengers on board, of which 128 were American. Creating more anti-German sentiment as a result of the sinking, the U.S. demanded reparations from Germany for the sinking and put an end to attacking unarmed passenger ships. Germany pledged to make sure passengers were safe before sinking any more unarmed ships. However, only three months later, in November 1915, another German U-boat torpedoed an Italian ship without warning, which killed 27 Americans who were on board.

The Zimmerman Telegram
In January 1917, a telegram sent by Arthur Zimmerman, the German foreign minister, to Mexico's German ambassador promised Mexico that, in exchange for its support during the war, it would help Mexico retake the U.S. territory it had lost during the Mexican-American War. British Intelligence intercepted the telegram and was able to decrypt the message, but it did not pass it along to the Americans right away. The British didn't want the Germans to know that they had broken their code. However, when Germany continued to attack American ships, the British used the telegram to persuade the Americans into joining the war on the side of the Allies. After the telegram was sent to President Wilson, the story became top news in the American press, helping to convince the U.S. public that war was inevitable. The U.S. officially entered the war on April 6, 1917. The Americans fought with the Allied forces from that point on until the war ended on November 11, 1918.