Both groups are called Slavs. In the years before World War I, Serbia wanted all Slavic peoples to form one empire, free from rule by the Turks or Austria-Hungary. The Serbs' wish to unite all the Slavs in one empire is an example of. imperialism.
To be eligible to serve, you must be at least 25 years old, been a U.S citizen for the past seven years, and be an inhabitant of the state they represent (at election time).
It must be approved! Hope this helps!