Nationalism is the feeling of belonging to a specific culture, region, language and people (or, in some nationalist arguments, to a race). In the case of Serbia, since the 18th century the desire to unite the Serbian language countries was strong - Great Serbia, but the presence of the Austro Hungary Empire was a threat to this nationalist idea. When Franz Ferdinand attempted to incorporate Serbia to his rising empire, nationalists such as Gravilo Princip fought against.
The US was certainly anticommunist and certainly a French ally but it is described in Bernard Fall's "Hell in a Very Small Place" as a gradual process.
