Pan-Slaavism, or Slaavic nationalism, was one of the reasons (the others being a desire for access to the Bosphorus and a need to avoid further humiliation after stepping down in the previous two Balkan Wars) why Russia supported their ally Serbia when threatened with the Ultimatum by Austria-Hungary, which was 10 times Serbia's size. Their mobilization in order to aide Serbia caused Germany to react by enacting the Schlieffen Plan, which involved Belgium, France, and by extension their ally Great Britain and its massive empire.

In otras words, Pan-Slaavism(partially) = Russian mobilization = Schlieffen Plan = expansion of the war from The Third Balkan War to the First World War.