germany
The main ally of Serbia was Russia
Serbia
Serbia wasn't allied with Belgium
Because Russia is serbia's ally
Austria.
Serbia was not allied with Germany; Italy and Romania, yes.
Russia declaring war on Germany to protect its ally Serbia.
Russia was Serbia's main ally during World War I. Russia had a powerful army and powerful allies as well.
The question doesn't specify, but I assume you're referring to World War One. The war began between Serbia and Austria-Hungary. Russia was an ally to Serbia, thus they jumped in to support their ally.
the colonies
yes it did, it also had a very strong ally, the soviet union
they were only defending their ally, Austria Hungary, which was attacked by Serbia