The document discusses the United States entry into World War 1. It states that the sinking of the passenger ship Lusitania by a German U-boat in 1915, which killed over 1,000 people including Americans, was a major event that led the US to join the war. Another key event was the Zimmerman Telegram, in which Germany attempted to ally with Mexico against the US. The document provides background on the countries involved in World War 1 and notes that over 2 million American soldiers fought after the US officially entered the war in April 1917.