It's important to know the difference between an alliance and an entente in order to answer this question. alliance means that when one enters a war, every ally of that country enters as well. entente means that when one enters a war, others are not required to. the Triple Alliance meant that once war started for Austria-Hungary, war also began for the Allied German States and Italy. however, although Serbia (where the Alliance was attacking) was "friends" with the entente, it was not a member of the Triple Entente--and because as an entente they were not required to fight for one another anyway--it was up to France, Britain, and Russia had the option to join or not. but all ended up joining because, to put it jokingly, "their friends are fighting our friends!"
