They are Slavics.
Russians and Serbs are Slavics.
Russia considered itself the protector and leader of all the Slav peoples, and Serbia was/is primarily a Slav nation, like the (European) Russians themselves
Russia helped Serbia when Serbia had rebelled
Because Russia is serbia's ally
Mobilized its military because of its alliance with Serbia.
Russia.
Germany, Austria/Hungary, and Italy by the Triple Alliance!!
Serbia assassinated arch duke Ferdinand Austria attacked Serbia Russia attacked Austria because they were allied to Serbia Germany attacked Russia because Germany was allied to Austria France and England were allies of Russia, so Germany attacked France France was then in war with Germany
Yes. Russia entered a regional conflict between Austria-Hungary and Serbia to protect its fellow Slavic nation.
Russia was compelled to aid Serbia against Australia.
Great Britain, France, Belgium, Russia. Russia backed Serbia. As Russia was allies with Britain and France through the triple entente, Serbia had their backing as well. Eventually, America and Belgium supported them too.
Because Germany declared war on Serbia, russia mobilized its army to protect Serbia due to an old alliance