United States Enter War! Today, we have word that despite earlier decisions, the United States of America has now decided to become part of the Great War. The States had first decided not to become part of the war, saying it was "strictly a European conflict." The states have entered the war for several reasons. These reasons include: German subs had a policy of warning and allowing passengers to evacuate. However, in 1915 the Lusitania was sunk, killing over 120 Americans. Many Americans were not happy about this at all. President Wilson took a stronger stance and issued a warning letter. The Germans responded by temporally ceasing submarine welfare.