One of the lesser-known conflicts of the nineteenth century was the Serbo-Bulgarian War. It was a war fought the resurrection of one state which hadn't seen independence since the fifteenth century. The Russo-Turkish War of about a decade before had sealed Bulgaria's independence, but it was threatened by the pervasive influence of Serbia in the Balkan peninsula. In a campaign that didn't lasted two weeks, the outmatched Bulgarian armies rolled up superior Serbian forces and threatened to completely rout their military. Austria-Hungary, bound to Serbia, sent envoys to tell Bulgaria to cease and desist, or Austria-Hungary would take action. In OTL, the cooler heads prevailed, and the war succeeded in preserving Bulgaria's independence from the neighboring powers who wished to undermine it.
My question is, what if Bulgaria had ignored the entreaties had kept going? If Austria-Hungary declared war, would Russia (historically Bulgaria's big brother) declare war in turn? Would that then cause the other two major powers--Italy and Germany--to make good on their promise in the Triple Alliance? What would Britain and France do, if anything? Or, would Russia even go to war at all?