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When Russia found out that Austria-Hungary was declaring war on Serbia, they came into alliance with Serbia for several reasons.
because they have to do it
The long term reasons for Russia's entry into WWI were to save face on the international arena by participating in the war and winning; unite the country with a very patriotic war fought for the ideal of pan-slavinism (help me out if that's not the correct term); perhaps territorial gain at the expense of her neighbors, Austria Hungary, Germany, and the Ottoman Empire; and financial assistance in return for military involvement from her Western allies. If anybody knows any other reasons--of which I'm sure there are more--feel free to add or edit this answer.
stuff and more stuff
well there wasnt an initail cause man. there were three long term reasons and one thing that set it off. the three long term causes i think was the arms race, territory battle and somethign else i cant remem. the thing that set it off though was the asasination of France Ferdinand. this is what i think anyway
they were blamed for starting the war
Serbia's organization, the black hand, was behind the assassination of Archduke Franz Ferdinand of Austria-Hungary, the heir to the throne. There were also many long-term causes leading up to this, but this was the event that set everything in motion
the long TERM effect was the CoLd WaR!!!!!!!!!!!!!!
There was no way Serbia could avoid Yugoslav war, because if there was Serbia would have done it...
Serbia was in WWI because Austria-Hungary declared war on them.
Serbia Serbia
If you mean how did alliances cause WWI, it was because when one country attacked another, each ally had to fight for the first, which made more people angry Serbia provoked Austria Austria attacked Serbia Russia was allied to Serbia, so they attacked Austria Germany was allied to Austria so they attacked Russia's allies, France and England