I think the only legitimate causus belli at the time was the crumbling Ottoman Empire's treatment of its Armenian minorities.
It's unfortunate that most people only recall the petty territorial disputes that occurred between the Central Powers and their Western foes, and forget that the most poignant moment of the war came when besieged Armenians in Van made a last ditch effort to save themselves from massacre at the hands of the Turkish military and their personally trained chetes.
All of the other issues that putatively sparked "The Great War"-with the possible exception of Serbian nationalism, which precipitated the fight in the first place-are petty when compared to what was occurring throughout Turkey during this period.
The massacre of the Armenians in 1925 was the first modern example of mechanized, universally applied ethnic genocide in the 20th Century. It set the example-not only in moral terms, but in terms of actual real world applications-for Hitler's execution machine.