The classmate post discusses three key causes of America's entry into World War 1: 1) The sinking of the passenger ship Lusitania by a German U-boat in 1915, killing over 100 Americans, without warning to the passengers; 2) Reports in the American press, some later found to be German propaganda, of unarmed civilians being killed and towns destroyed; 3) Germany's resumption of unrestricted submarine warfare in 1917, torpedoing passenger ships in spite of promises to end the practice.