What was The new nation that included the prewar nation of Serbia was?

The new nation that included the prewar nation of Serbia was Yugoslavia. Established after World War I in 1918, it united several South Slavic groups, including Serbs, Croats, and Slovenes, into a single state. Yugoslavia aimed to create a sense of national unity among its diverse ethnic groups, although this would later lead to significant political and ethnic tensions. The country existed in various forms until its eventual dissolution in the 1990s.