Which side did the US join out of the 3 powers of ww1 and why?

Answer 1

first of all the U.S didn't join anyone the allies powers joined them.

Answer 2

America wanted to remain nuetral but the central powers kept attacking america for sending supplies to the allies and after Germany sank the lucitania and kept harrassing Mexico to join the war, america had to go and defeat the central powers. The allies wanted to get into an alliance with america but america refused to let them help because they had done so badly. America won the war against the central powers single handedly and the allies used america for protection.