Answer:
Main causes of WW1 were imperialism and militarism. Namely, members of Central Powers in Europe were not satisfied with the distribution of colonies in the world. Most of the colonies were in the hands of members of Triple Entente. Therefore, countries started spreading their military power, which eventually led to war. United States were helping Allies by sending goods via ships and from 1917 they actively participated in war on their side.
Step-by-step explanation:
Rise of imperialism was the main reason for WW1. Germany and Austria-Hungary were especially dissatisfied. Therefore, when archduke Franz Ferdinand was killed in Sarajevo this was used as a pretext to start the war.
US officially didn't join the war efforts before 1917, but were helping Allies, especially Britain before the war started.