World War I started in 1914.  The United States did not take either side at that point.  The US did not get involved in the war until 1917, when it entered the war on the side of the Allied Powers.

At the beginning of the war, the United States did not want to get involved.  There were two main reasons for this.  First, the US did not feel as if the war would impact it.  The war was happening on the other side of the Atlantic.  There was no way that either side was going to attack the United States.  Therefore, there was no real reason for the US to become involved.  Second, the US was (and is) a country of immigrants.  There were many Americans who favored the Central Powers at the start of the war.  There were many German immigrants who felt ties to their ancestral home.  There were many Irish immigrants who hated the English.  On the other hand, the US had close ties to England and was, of course, culturally very close to the English.  In short, there were real reasons to support each side in the war.  For these reasons, it made more sense for the US to stay neutral.  That is what the US did at the start of the war.