There were many reasons why the United States joined World War I. The first set of reasons deals with actions of our enemies, mainly Germany, towards us. The Germans were violating our rights to trade with other countries. According to the agreed upon rules of war, a neutral country can trade with countries at war as long as they aren’t trading war materials. If a country at war wants to stop a ship from a neutral country and search it, it may do that. It can’t sink a ship without warning. When Germany began sinking our ships without warning after promising not to do this, President Wilson eventually went to Congress to ask for a declaration of war. Germany also tried to get Mexico to attack us. When the Zimmerman note was made public, Americans were furious at the Germans. Germany believed Mexico might want to help them in exchange for getting back some of the land Mexico lost in the Mexican-American war.
Another set of reasons why we went to war was idealistic in nature. For example, we wanted to make this war the last war ever. We also wanted to make the world safe for democratic governments. Unfortunately, these goals were not fully achieved over the long run.