Answer:
Main causes of WW1 were imperialism and militarism of great powers. The main reasons why United States entered the war was the sinking of ship Lusitania and the famous Zimmerman's affair.
Step-by-step explanation:
At the beginning of 20th Century European powers were divided in two military alliances - Entente and Central Powers. Members of the second one believed that colonial possessions were not divided as they should have been. That is why most of countries, knowing that war is inevitable started strengthening their military. At the beginning United States tried to stay out of conflict, but after the sinking of ship that carried American citizens and a telegram from Germany to Mexico, where they asked for their involvement in the war against United States changed the attitude of US.