The main ally of Serbia was Russia
Serbia
Serbia wasn't allied with Belgium
Because Russia is serbia's ally
Austria.
Russia declaring war on Germany to protect its ally Serbia.
Serbia was not allied with Germany; Italy and Romania, yes.
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
Austria-Hungary believed that Russia would not come to the aid of their ally Serbia because they thought Russia was preoccupied with internal issues and would not risk a war with Austria-Hungary.
yes it did, it also had a very strong ally, the soviet union