Final answer:
World War I lasted for nearly three years before the U.S. entered. It began in 1914 and the U.S. joined the fight in April 1917, which means the U.S. participated actively for about a year and a half until the Armistice.
Step-by-step explanation:
World War I was underway for almost three years before the United States entered the conflict. The war started in the summer of 1914, and it was not until April 1917 that the U.S. declared war on Germany. This decision came after a series of events that shifted American public sentiment, including the sinking of ships carrying American civilians by German U-boats. President Woodrow Wilson had hoped to maintain neutrality, but with the war's escalation and the direct impacts on American interests, intervention became inevitable. The American Expeditionary Force did not begin arriving in large numbers until the summer of 1918, which means that the U.S. was actively involved in the war for about a year and a half until the Armistice in November 1918.